We are upon WebOb 1.7

Closes #288

__call__ and conditional_response_app both wanted a copy of the
headerlist anyway, so iterate and if we stumble upon a Location header,
make it absolute.

If the user provides a headerlist, Response.__init__ will no longer
attempt to set the default Content-Type, nor will __init__ add a charset
to any existing Content-Type.

The Content-Type will only be set if no headerlist is provided, and the
HTTP status code allows for a body. The charset will be set only if
there is no charset on the Content-Type provided.

Response(headerlist=[], content_type='text/plain', charset='UTF-8')

will have a headerlist that is: [('Content-Length', '0')]

Response(status='204 No Content')

will have a headerlist that is: []

__init__ will now also ignore body/app_iter if the status code has no
body.

Response(status='204 No Content', app_iter=[b'test'])

Will not have a Content-Length, and .body will return b''. If you would
like to violate HTTP standards, you may of course do:

resp = Response(status='204 No Content')
resp.body = b'test'

__init__ won't let you create an invalid resonse, what you do with the
returned response however is up to you.

This is mainly used to allow users to continue using .text even with
Content-Types that don't have a charset.

Looking at you Pyramid...