Allow to override how headers are encoded

Motivation:

Even if its against the HTTP RFC there are situations where it may be useful to use other chars then US_ASCII in the headers. We should allow to make it possible by allow the user to override the how headers are encoded.

Modifications:

- Add encodeHeaders(...) method and so allow to override it.

Result:

It's now possible to encode headers with other charset then US_ASCII by just extend the encoder and override the encodeHeaders(...) method.