Make it easier to write POST test requests.
Introduce the EnableRelaxedContentType option

By default the Content-Type is now validated, and the option
EnableRelaxedContentType can be set to disable this check.

       // If true, the handler does NOT check the request Content-Type. Otherwise, it
       // must be set to 'application/json' if the content is non-null.
       EnableRelaxedContentType bool

Fix Issue #5