The body is ignored unless the Content-Type is ``application/x-www-form-urlencoded``, according to [section 3.4.1.3.1 of RFC 5849](http://tools.ietf.org/html/rfc5849#section-3.4.1.3.1), which describes what goes into the signature base string.

I'm setting ``''`` because it seems a sensical "null" value for a string. Also, [this line](https://github.com/idan/oauthlib/blob/0.3.7/oauthlib/oauth1/rfc5849/__init__.py#L729) of code in oauthlib makes the same substitution under the same conditions.

The body variable is going to be ignored anyway. This code now avoids the exception that arises from decoding it.