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
Just to be more explicit about the fact that we are checking whether these variables were set at all, not whether they were set to an empty string by the caller. Also, it is a tiny bit faster to do an "is" check like this than a truthy check.