Always set encoding to utf-8 for better performance

Thanks a lot for version 1.7!

Not setting the ```response.encoding``` to a fixed value causes the requests library to make a costly guess of the encoding (the first time ```response.text``` is accessed but even for json it can be utf-8, utf-16 ...).

This leads to a factor three in time needed to get a document from CouchDB. I haven't realized that in the first place. So this fix gives a small performance boost.