Previously, parse_header_string was only called with data coming out of
requests, which would be either bytes or unicode. Now that we're sending
it request headers as well (see related change), we need to be more
defensive.
If the value given is neither bytes nor unicode, convert it to a native
string. This will allow developers using the client API to continue
sending header dicts like
{'X-Delete-After': 2}
...as in Swift's test/probe/test_object_expirer.py
Change-Id: Ie57a93274507b184af5cad4260f244359a585f09
Related-Change: I43dd7254f7281d4db59b286aa2145643c64e1705