Add raw_headers to both client and server API

[asvetlov]

headers are utf-8 surrogate escaped now.
raw_headers should be bytes -- encoded version doesn't allow restoring original bytestring back if encoding is not utf-8

[asvetlov]

Done.
Personally I don't have a feeling that we need raw_headers in client request and especially in web.Response.
If somebody need it -- please open a separate issue.

[lock]

This thread has been automatically locked since there has not been
any recent activity after it was closed. Please open a new issue for
related bugs.

If you feel like there's important points made in this discussion,
please include those exceprts into that new issue.