Check media type of Link Description Object for Response Validation

#### Changes
With this PR, `ResponseValidator` will skip validation of json when mediaType of Link is other than JSON.

#### Notes
According to http://json-schema.org/latest/json-schema-hypermedia.html#anchor33, Link Description Object can have "media type" for specifying the media type of response.
`ResponseValidator` should skip validation when media type is explicitly described as non-JSON in schema.json. 

Since the default mediaType is "application/json", it doen't affect the existent applications.

ref: https://github.com/brandur/json_schema/pull/14