Set content-length despite webob

Webob has a tendency to wipe out the content-length we set on responses.
This causes eventlet to use chunked transfer encoding. By changing the
order of operations, however, we can trigger eventlet to use the simpler
and more informative Content-Length header.

Change-Id: I0ed27c9e668a538bc0fc29cb6e9127dfb7fd8c78