Fix sending JSON examples

[maxlinc]

This fixes a bug reported by @rafaelportela. Pacto wasn't properly sending
JSON request examples if they were written as a JSON object in the contract. It may
have sent them if they were written as a JSON string.

This introduces rudimentary support for media types, but probably should be just a
temporary solution until a more extensible system is created for #154.

[maxlinc]

This should probably have a test case before it gets merged. I've got a sample contract and API from @rafaelportela that I was using to test. That scenario could be added after refactoring so its more in line w/ other samples.

I'd do it now, but I've got to catch a bus very early in the AM. So 🌃 for now.

[coveralls]

Coverage decreased (-5.21%) when pulling 90f5035 on maxlinc:fix_sending_json into 9c47be2 on thoughtworks:master.

[coveralls]

Coverage increased (+0.15%) when pulling 4b2f068 on maxlinc:fix_sending_json into 74d11e0 on thoughtworks:master.

[rafaelportela]

It works perfectly with my examples! =]