Rename enum evhttp_connection_error -> evhttp_request_error

And all fields in that manner
Comment evhttp_request_error enum.
Add test for EVREQ_HTTP_REQUEST_CANCEL into http_cancel_test()
Merge remote-tracking branch 'git@github.com:azat/libevent-deb.git/pass-http-client-error-codes-to-callback' into 2.1-testing
Updated debian/changelog

See
https://github.com/azat/libevent/tree/pass-http-client-error-codes-to-callback
for more info