Fix parsing the media type in GameServerAllocation

[aLekSer]

Add a call to filter out optional parameters send in an HTTP header.

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking

/kind bug

What this PR does / Why we need it:
There are parameters like charset which would coexist in Content-Type header. That's lead runtime.SerializerInfoForMediaType to fail, because unexpected parameter was provided.

Which issue(s) this PR fixes:

Closes #1748 .

Special notes for your reviewer:
Thanks @markmandel for providing the solution in the issue comment

Steps to reproduce:
Proof of the fix:

export APISERVER=localhost:8001
$ kubectl proxy &
$ curl -X POST -H 'Content-Type: application/json; charset=utf-8' --data '
{
   "apiVersion": "allocation.agones.dev/v1",
   "kind": "GameServerAllocation",
   "spec": {
      "required": {
         "matchLabels": {
            "agones.dev/fleet": "simple-udp"
         }
      }
   }
}
' ${APISERVER}/apis/allocation.agones.dev/v1/namespaces/default/gameserverallocations
{"kind":"GameServerAllocation","apiVersion":"allocation.agones.dev/v1","metadata":{"name":"simple-udp-xv6d5-g4zsd","namespace":"default","creationTimestamp":"2020-08-13T09:05:15Z"},"spec":{"multiClusterSetting":{"policySelector":{}},"required":{"matchLabels":{"agones.dev/fleet":"simple-udp"}},"scheduling":"Packed","metadata":{}},"status":{"state":"Allocated","gameServerName":"simple-udp-xv6d5-g4zsd","ports":[{"name":"default","port":7870}],"address":"34.82.245.95","nodeName":"gke-test-cluster-default-dfffabe4-t97g"}}

[agones-bot]

Build Succeeded 👏

Build Id: 45774ee4-5914-41d3-9c79-d5c00056d229

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.8.0-a3bd965
• image: gcr.io/agones-images/agones-sdk:1.8.0-a3bd965
• image: gcr.io/agones-images/agones-ping:1.8.0-a3bd965
• Linux C++ SDK (build): agonessdk-1.8.0-a3bd965-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.8.0-a3bd965.zip

A preview of the website (the last 30 builds are retained):

• https://a3bd965-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1749/head:pr_1749 && git checkout pr_1749
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.8.0-a3bd965

[markmandel]

can we errors.Wrapf(....) this error message before passing it back out, so we can capture context?

Outside of that, this looks good to go.

[aLekSer]

Tested latest version:

 curl -X POST -H 'Content-Type: application/json; charset=utf-8' --data '
{
   "apiVersion": "allocation.agones.dev/v1",
   "kind": "GameServerAllocation",
   "spec": {
      "required": {
         "matchLabels": {
            "agones.dev/fleet": "simple-udp"
         }
      }
   }
}
' ${APISERVER}/apis/allocation.agones.dev/v1/namespaces/default/gameserverallocations
{"kind":"GameServerAllocation","apiVersion":"allocation.agones.dev/v1","metadata":{"name":"simple-udp-pb8kx-v4fq8","namespace":"default","creationTimestamp":"2020-08-13T17:11:29Z"},"spec":{"multiClusterSetting":{"policySelector":{}},"required":{"matchLabels":{"agones.dev/fleet":"simple-udp"}},"scheduling":"Packed","metadata":{}},"status":{"state":"Allocated","gameServerName":"simple-udp-pb8kx-v4fq8","ports":[{"name":"default","port":7042}],"address":"34.82.245.95","nodeName":"gke-test-cluster-default-dfffabe4-t97g"}}

[agones-bot]

Build Succeeded 👏

Build Id: a61ac52f-5d59-4072-9e53-0f0ddf02e5d9

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.8.0-ac94465
• image: gcr.io/agones-images/agones-sdk:1.8.0-ac94465
• image: gcr.io/agones-images/agones-ping:1.8.0-ac94465
• Linux C++ SDK (build): agonessdk-1.8.0-ac94465-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.8.0-ac94465.zip

A preview of the website (the last 30 builds are retained):

• https://ac94465-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1749/head:pr_1749 && git checkout pr_1749
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.8.0-ac94465

[markmandel]

Fantastic!

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aLekSer, markmandel

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