Allow the user to specify his own custom parsing block on the POSTed body

The block should be submitted to the use statement and takes one argument: the body of the request.

Example:

  use Rack::PostBodyContentTypeParser do |body|
    { payload: JSON.parse(body) }
  end
Merge branch 'allow_post_body_content_custom_parsing' of github.com:libis/rack-contrib into allow_post_body_content_custom_parsing