x-www-form-urlencoded parameters are cached so the Refresh button produces a bad oauth signature

[steventucker]

To reproduce the problem:

• Set up a POST with the header Content-Type: application/x-www-form-urlencoded
• Set up OAuth authorization with fixed nonce and timestamp
• Note the OAuth signature
• Add a parameter to the body e.g. "a=b"
• Refresh the signature
• Change the body back to what it was
• Refresh the signature. It should come out to the same as before, but it doesn't because it has remembered the "a=b" parameter