Don't double-decode the request parameters. When the server already decodes
the x-www-form-urlencoded request correctly. By decoding it again, we fail
to "decode" certain special characters, like % and + (with the latter being
silently ignored and the former throwing an exception).
Fixes #1.
Let's not have a encode/decode pair around the set/getHeader functionality
if we only need to encode our values before sending them off to the server.