Use Omnipay logic to do redirects in GatewayResponse

They're not always GET urls, but can be POSTs through a generated form
Merge commit 'd816de9f02c3f4fd087a5d3a87fe6e61b72cf0e8'

* commit 'd816de9f02c3f4fd087a5d3a87fe6e61b72cf0e8':
  Use Omnipay logic to do redirects in GatewayResponse

Conflicts:
	code/GatewayResponse.php