Fill the send buffer with smaller strings in order to more completely fill it.

This makes the test pass on OS X.  Previously it failed with the
wrong exception type probably because there was enough room left
in the send buffer for the handshake to start.
Make sure we still write a lot of bytes, too.
Merge pull request #31 from pyca/osx-wantwrite

Fix the WantWriteError test on OS X (purely a test bug).