add auto detect charset from http body when http headers not seted

requests detect page's charset/encoding from responsed http headers, but many servers won't set it in headers.

In this case, requests will set the charset/encoding to the default 'ISO-8859-1', and result in decoded error, many Chinese developers get lose and complain.

For example:

    print requests.get('http://www.126.com/').text

This commit will detect charset/encoding from http headers, if no, from http body, for most situations, everything goes well now.