Only query values should be used for returnto

The addReturnTo method was using all submitted values, GET and POST,
rather than just the GET parameters.  This could lead to undefined
behavior as form fields are resubmitted as GET parameters when
returning.

Adjust to only use the query parameters when returning.

Change-Id: I4f4a3767b7ac67a7db16356bc3146b08bb93c2c6