Added utf8 in curl query parameters

There is a problem if GET parameters contain non-ASCII symbols. `/silk/request/ID/`throws error 500 and complains on `urlencode`.

That's known issue with `urlencode` - it expects values in dict to be string, not unicode. So I've added explicit utf8-encoding here.

Tests for CURL are all commented out, so no test here. I can add it as a separate `TestCase`, if necessary.