Respect Content-Type instead of Accept header in consume!

This one fixes #64 .  Today `consume!` incorrectly uses `Accept` request header to parse incoming content. The `Accept` header is about response data format not incoming format. This fix changes `consume!` to use `Content-Type` of request header to determine format of consumed content. If format is missing, unknown or not registered with `Mime::Type` the rack based framework will return HTTP status `415 Unsupported Media Type`.
So is that a "bug" in Rails AC::TestCase? Passing `:format` doesn't seem to set the format right (Content-type)?