Modify http data string sanitizing to account for cookie delimiter

When sanitizing http data strings, a `&` was treated as the only possible delimiter, while cookies use `;` instead. This accounts for the cookie delimiter as well.

This assumes that a string is the proper type for cookie data in the http interface, as sent by the tornado client in https://github.com/getsentry/raven-python/blob/5.1.1/raven/contrib/tornado/__init__.py#L191 and shown in http://sentry.readthedocs.org/en/latest/developer/interfaces/#sentry.interfaces.http.Http. The previous tests indicate it's a dict instead.