fix(HTTPError): Remove dependency on blank title to determine serialization

This patch attempts to make the code easier to understand and maintain
going forward by removing the dependency between the title attribute and
the "serializable" property. The "serializable" property was renamed to
be more indicitive of the context in which it is actually used.

Closes #265
IIRC, it was done to avoid introducing a breaking change in the case that existing apps were expecting this error to return a body. But now that you mention it, 0.2 may be a good time to make that change and just not accept any kwargs.