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
OK, I'll go ahead and submit a separate PR for that. Thanks for the review! It is really useful to get a fresh pair of eyes on this code.