Only set auth_path once, fixes signing errors

Fixes aws/aws-cli#986

The bug was that the fix_s3_host always sets the auth_path
to the current path of the request.url.  However, in the case
of a retried request, we call fix_s3_host again which changes
the auth_path to the (already modified) request path.  This
generates an auth error.

The fix now is to check if auth_path is set, and if it is, then
we can immediately return.

cc @kyleknap @danielgtaylor 
``BaseSigner`` is not used in this file.