Fix incorrect return impl in SmileXContent.createParser.

Close #5510
Use build-in function to write field name in JsonXContentGenerator#writeRawField

The #writeRawField method forcefully writes a `,` spearator expecting a raw field
to never start as the first value in an object.

Closes #5514
Randomize XContentType for requests

The used XContentType is static and should be randomized per test.

Closes #5515