if no custom parser provided for a media type that uses +json suffix, default to application/json parser

this is a follow up of the work done by @ericmedem and @jxson for issue #121 bringing it up to speed with the current master.

if you're using a media-type that uses a +json suffix and don't provide a custom parser and enable buffering then the default should be to parse it as application/json so that it just works out of the box for application/hal+json and other media types that indicate their representations can be parsed as json by including the +json suffix.