This fixes #9 and #8 by reading headers_out from the subrequest resposne
and copying them into the main request's headers_out.  Previously, the
direct assignment (shallow copy) produced the effect whereby no filters
that ran after the shib module could modify headers.

The reason for this appears to be linking inside the ngx_list
``headers`` inside the request option; suffice to say, this is a much
better approach.