follow redirects(up to 3 times) for post requests in mailgate

in case http server is configured to redirect http requests to https requests
or something like that.

we can't simply "push @{ $ua−>requests_redirectable }, 'POST';" because for a
302/303 redirect, LWP performs an empty GET request instead of re-POSTing the
original content. see also #14114
Merge branch '4.2/mailgate-post-redirect' into 4.2-trunk