Fix dict(HTTPHeaderDict(other_dict)) and deal with changes in how we
implement the HTTPHeaderDict as a MutableMapping subclass instead of
as a dict subclass.

Now when we send data with the HTTPHeaderDict we will use the original
casing, e.g., if we create an instance with

    {'COOkie': 'value'}

Then

    dict(HTTPHeaderDict({'COOkie': 'value'})) == {'COOkie': 'value'}

So we should expect that the returned headers preserve the capitalization
that was provided to the HeaderDict.