Skip validation of request body when content type is not application/json

According to [draft-04 of JSON Hyper-Schema](http://json-schema.org/latest/json-schema-hypermedia.html#anchor37), the server should accept non-json request. 

In this PR, I suggest to skip the validation of json format when `Content-Type` of the client is not `application/json`. 

Also, when the request body is not json, the schema validation will be skipped, though It is desirable to validate paramters on other standard media types including `text/plain`, `multipart/form-data` and `application/x-www-form-urlencoded`.