Fix posting non-ascii data via httplib[2]

For python2:
Headers and http method had been unicode (because of `__future__  unicode_literals`), but location and body — string (bytes).
`httplib2` uses `httplib` inside, `httplib` tries to concatenate headers and strings, decoding payload bytes with default (ascii) encoder. `UnicodeError` happened.
I've converted all data to string, `httplib` sends it as is.

Python3:
`httplib` just works with unicode data everywhere. Everything is OK.

As usual unittests haven't become worse, but still didn't pass.