Add success action status to PresignedPostPolicy

[sinhaashish]

Parameter added to set success-action-status for making PresignedPostPolicy request.

The generated curl command :

curl -X POST  -F bucket=testbucket -F x-amz-date=20190804T172411Z -F success_action_status=201 -F x-amz-signature=c9358c3dc8c5e23b451c1a248f6523e44daeb1a01bd63999b3f389e6a8345fc4 -F key=my-objectname -F x-amz-algorithm=AWS4-HMAC-SHA256 -F Content-Type=image/png -F x-amz-credential=Q3AM3UQ867SPQQA43P2F/20190804/us-east-1/s3/aws4_request -F policy=eyJleHBpcmF0aW9uIjoiMjAxOS0wOC0xMVQxNzoyNDoxMC4xNzNaIiwiY29uZGl0aW9ucyI6W1siZXEiLCIkYnVja2V0IiwidGVzdGJ1Y2tldCJdLFsiZXEiLCIka2V5IiwibXktb2JqZWN0bmFtZSJdLFsiZXEiLCIkQ29udGVudC1UeXBlIiwiaW1hZ2UvcG5nIl0sWyJlcSIsInN1Y2Nlc3NfYWN0aW9uX3N0YXR1cyIsIjIwMCJdLFsiZXEiLCIkeC1hbXotYWxnb3JpdGhtIiwiQVdTNC1ITUFDLVNIQTI1NiJdLFsiZXEiLCIkeC1hbXotY3JlZGVudGlhbCIsIlEzQU0zVVE4NjdTUFFRQTQzUDJGLzIwMTkwODA0L3VzLWVhc3QtMS9zMy9hd3M0X3JlcXVlc3QiXSxbImVxIiwiJHgtYW16LWRhdGUiLCIyMDE5MDgwNFQxNzI0MTFaIl1dfQ== -F file=/home/ashish/Pictures/test.png https://play.min.io:9000/testbucket 

The XML received :

<?xml version="1.0" encoding="UTF-8"?>
<PostResponse><Bucket>testbucket</Bucket><Key>my-objectname</Key><ETag>&#34;d1237fb44ada3718026aeafc725e8402&#34;</ETag><Location>https://play.min.io:9000/testbucket/my-objectname</Location></PostResponse>

Closes #782

[harshavardhana]

How about we update the docs in API.md as well.

Rest of the code is working LGTM

[sinhaashish]

How about we update the docs in API.md as well.

Updated it.

[poornas]

LGTM

[poornas]

@sinhaashish , travis failures on this.

[sinhaashish]

@sinhaashish , travis failures on this.

restarted the job and it passes.

[kannappanr]

ping @harshavardhana