CreateFormPostUri can cause subtle bugs

[carolynvs]

CreateFormPostUri takes a Uri for the redirect value and if the value supplied does not have a file extension, e.g. http://example.com, Uri will quietly change it to http://example.com/. This easily leads to invalid signature exceptions because the redirect value in the form and hash are different.

Since nothing is gained by using a Uri as a parameter, I suggest switching to string when we revisit Object Storage.