#65 Support for 405 method not allowed HTTP status code

[Gabriele Bartolini]

When performing a HEAD call before GET, in a case
where the server does not allow the HEAD method, the
library net does not retrieve automatically the document
(trusting the users' settings).

A little bit more of intelligence is needed, in particular by
adding in HtHTTP::Request() an 'if statement' for the
case when the server returns a 405 status code;
logically, when a HEAD method is issued and a "not
allowed method" response is given back, a GET of the
resource should be performed.

A proper Document Status for this error should be add in
the Transport class (Document_method_not_allowed?).