This is a workaround for https://github.com/webrecorder/warcio/issues/90

As a consequence, SpecDependencies.extra can now be any data type that can be put into JSON; unhashable types previously caused a crash due to the lru_cache.

It is rarely necessary to access the history, and the tuple return value clutters the spec file code.

As a consequence, it's no longer possible to return None if an error occurred without losing the history.
To replace that, this also introduces a DummyClientResponse, which is kind of ClientResponse-like, has the same qhistory attribute, and evaluates to False when cast to bool (such that the intuitive `if response` works as expected).

For example, if the connection breaks while retrieving a response but after the headers have been parsed, the response body would be incomplete.