Fixes the do not follow redirects bug

The HttpEngine would not obey the set state because there was no
correlation between the HttpURLConnection.setInstanceFollowRedirects
method and the OkHttpClient.

Fixed the missing link by adding a new setFollowRedirects method to the
OkHttpClient class.  Bridged the gap for the HttpURLConnection.setInstanceFollowRedirects
by forwarding that state into the OkHttpClient.

Now the HttpEngine will always obey the OkHttpClient redirect state when
attempting the followUp phase.

Added necessary test to both OkUrlFactoryTest and CallTest.

https://github.com/square/okhttp/issues/943
I think I might tweak this sentence later once this is landed. I thought in OkHttp following redirects was the default.