1.3: req.headers dictionary to contain bytes instead of str, and be optional at all

[pfalcon]

Currently, for each request, headers are being parsed, decoded to str's and stored in req.headers dict. The idea is to skip decoding to str's to save on memory pressure. But a lot of request handlers don't care about headers, so add a mode when headers aren't stored, but skipped instead. There also can be a need for request handler to parse headers itself, so allow that too. Allow to set the default header parsing mode for the WebApp (the default default will be "skip headers"). Finally, factor out header parsing to a method, to allow subclasses to override it.

[pfalcon]

Finally, factor out header parsing to a method, to allow subclasses to override it.

Done in d9840b6 , went into 1.2.3.

[pfalcon]

The idea is to skip decoding to str's to save on memory pressure.

In 1.3.