Strip whitespace off an IP address.
Proxy is passed an IPAddress; no need to double-convert it.
Update tests.
Add tests on ProxyConfig.__init__().
Make sure to eliminate empty entries in useragents.
Add returns for testing purposes

For testing purposes, ProxyConfig.__call__() now has return values.
Walk the proxy list.
If all proxies are valid, remote the header entirely.
Complete tests on proxy processing.
Start tests for middleware.py

Includes the full test suite for BarkMiddleware and a first test
case for bark_filter(), which is more complicated anyway.