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
Seems reasonable to have a followup patch removing it; I didn't have any objections but just wanted to understand how it was intended to be used.

If this is as good a time as any to remove it, I think it's less implicit behavior which is a good thing!