Fix incorrect marshalling of DismissalRestrictionsRequest field.

[ethomas2]

Previously, an empty DismissalRestrictionsRequest was being marshalled
to an empty array [] instead of an empty object {}. This is an invalid
api call according to github's documentation, and caused github to
respond with a 422.

Fixes #664

[ethomas2]

According to github's docs, to disable dismissal_restrictions, you are supposed to pass an empty object {}. go-github was passing an empty array [], which caused github to respond with a 422

It looks like this mistake was made in go-github because there used to be a typo in github's documentation that said to pass in an empty array. That typo has since been fixed in docs. I believe passing an array instead of an object has always resulted in a 422, even when the docs were incorrect.

[ethomas2]

Somewhat related to this pull this pull request:

There are several array-fields in structs for branch protection requests that get marshaled to nil instead of []. Github responds with a 422 when these fields are nil. For example

	_, _, err := ghclient.Repositories.UpdateBranchProtection(
		ctx, "MyOrg", "myrepo", "master",
		&github.ProtectionRequest{
			RequiredPullRequestReviews: &github.PullRequestReviewsEnforcementRequest{},
			Restrictions:               &github.BranchRestrictionsRequest{},
		})

gets this response from github

PUT https://git.dev.box.net/api/v3/repositories/3333/branches/master/protection: 422 Invalid request.

No subschema in "anyOf" matched.
For 'properties/users', nil is not an array.
For 'properties/teams', nil is not an array.
Not all subschemas of "allOf" matched.
For 'anyOf/1', {"users"=>nil, "teams"=>nil} is not a null. []

Instead you must explicitly set Users and Teams to be an empty array instead of a nil array

	_, _, err := ghclient.Repositories.UpdateBranchProtection(
		ctx, "MyOrg", "myrepo", "master",
		&github.ProtectionRequest{
			RequiredPullRequestReviews: &github.PullRequestReviewsEnforcementRequest{},
			Restrictions: &github.BranchRestrictionsRequest{
				Users: []string{},
				Teams: []string{},
			},
		})

There are more structs other than github.BranchRestrictionsRequest which follow this pattern. I've read go-github's docs for these fields and I realize this behavior is intentional, but I think it provides a poor experience. In my own project, i've resorted to writing my own branch protection structs that marshall nil arrays to []. Would you guys be open to me opening another pull request that marshals nil arrays to []?

[dmitshur]

This overlaps with #791.

[ethomas2]

I think #791 makes this pr unnecessary. Closing