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'
CSS file paths for default errors are apended to request.uri_base

With Dancer2 apps being able to be mounted via URLMap without adding
a prefix into the application, ensure the default error content also
adjusts paths to their css files appropriately.

Includes a simple test case. Also update dispatcher.t and dancer-test.t
so there is sufficient content in their request envs to calculate uri_base
to shhh use of uninitialized var warnings.
Update Changes
Merge branch 'veryrusty-Pr/relocatable_default_errors'
Work in progress for #316. Fixed spelling and grammar issues. Fixed
invalid code. Added a better code sample for sessions and logging in.
Update changes
Fix some params->{} with param('...')
Merge branch 'gideondsouza-pr/fix_cookbook_pod'
Fix Gideon D'souza name