Avoid unnecessarily decoding body data.
Avoid unnecessarily decoding body data.

In one case we had, the request body contained JPEG data with UTF-8
violations, causing exceptions.

Since the body is only part of the OAuth signature if it's
www-form-urlencoded, avoid decoding it in other cases.

ib-lundgren/flask-oauthprovider,aguegu/flask-oauthprovider,aguegu/flask-oauthprovider