Remove before hook from Plugin::Ajax.

Unlike D1, there is only one way to defined an ajax route in D2, so we
CAN specify the *response* content type in the ajax route code instead
of a before hook. (cf Dancer1 commit 3dd7224)

Also fix a typo where the request content_type was being altered, remove
trailing whitespace, and apply doc patches and config options added to
Dancer::Plugin::Ajax since the initial fork.

Closes #141.
Update ajax plugin tests.

Check that content_type header is not alteres for non-ajax routes.
Check content_type header can be set via configuration.
Update changes
Merge branch 'veryrusty-Pr/ajax_content_type'