Coerce the Content-Length of the custom error message to a string, not an
integer. Some pure Python WSGI servers aren't strict and violate the WSGI
specification by automatically converting the value to a string. Apache,
however, *is* strict, and considers this a 500 Internal Error
Fixes bug 1306963
Change-Id: I9b82ceee096b00c21c3b230dc67701bc40629968