Handle servers specified as unicode literals

So far `ES._check_server()` checked whether the given server was an instance of
`str`. This fails in case the server was specified as a unicode literal.

This commit checks whether the given server is an instance of `basestring`
instead of `str` to cover `unicode` objects as well.
Merge pull request #377 from 0x64746b/feature/handle_unicode_on_connecting

Handle servers specified as unicode literals