Enable skip_host and skip_accept_encoding

[kanzure]

Py2.4 added skip_host and skip_accept_encoding to httplib, but it's only
available when manually sending data over the wire with connect() and
putheader().

In httpconnection.py, httplib and http.client are wrapped with a small
fix to enable support for skip_host and skip_accept_encoding.

Also unit tests.

[shazow]

Hmm could you talk about when this is necessary?

First I've seen this request and I'd rather avoid adding too much niche functionality in the core library.

[kanzure]

There are some finnicky servers out there that I have encountered that completely explode when they are given an "Accept-Encoding" header. Obviously, these servers are wrong, but I would still very much like to use Requests.

[shazow]

Doesn't sound like @kennethreitz wants to include this in Requests either way.

I like having complete control of headers but perhaps we're better off reserving this for when we drop dependency on httplib (#58). I feel the pull as is adds more mess and complexity than the functionality is worth at this time. :)