[FEATURE] HTTP method tunneling
This change allows the HTTP method to be overridden by the client.
This is especially useful for clients that only support a subset of the
supported methods (e.g. browsers).
The request method can be overridden by specifying a ``__method``
argument. Additionally Flow now respects the ``X-HTTP-Method``
respectively ``X-HTTP-Method-Override`` header.
<input type="hidden" name="__method" value="DELETE" />
The HTTP request method can only be changed for POST requests
Releases: master, 2.0