Ensure that headers are returned as strings, not integers.

The 'Retry-After' header was set as an integer, and it was causing that
mod_wsgi failed with a 500 error, instead of the actual error. This
ensures that the headers are converted to str.

Fixes bug 1155585
Change-Id: If1b1de2308ccdc6f0588c8df5dc896fe5f767dbe
Merge "Ensure that headers are returned as strings, not integers."