Added failing test for issue #17: Validate that data is being sent as application/json
Added validation of mutation data content-type

returns 415 if it's not application/json

Fixes issue #17
Only check content-type if there is actually content.
Updated tests to set content-type on mutations.
Merge pull request #25 from brikteknologier/reject-non-json

Reject non json. Closes #17