pass kwargs to request handler's initialize method

hi again :)

i know that the main purpose of RequestHandler.initialize() method is to be overridden in derived classes, however, it would be nice for the base method to receive the kwargs argument as it is documented in the docstring.

this is useful (and i'm using it this way), when the requesthandlers are dynamically chosen and some of them overrides initialize() while others don't. With the current code, an exception is raised because the empty method doesn't expect arguments.

also, this is exactly how the code used to be just a couple of months ago (and the doc still report it that way), and i believe it is the correct behaviour

thank you,
flavio