Specify encoding for key-value parts of urlencoded HTTP query body

Specify encoding of keys and values that will be urlencoded and used as HTTP body for requests to Overpass API. Without specifying encoding it (on some systems only?) sends xml query in wrong charset causing names of persons with non-ASCII characters not found.
Merge pull request #12 from kolen/master

Specify encoding of keys and values of urlencoded body - fix for non-ascii person names in search