Fix a response header bug in the error middleware

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

ceilometer/api/middleware.py

@@ -124,7 +124,7 @@ class ParsableErrorMiddleware(object):
                 except ValueError as err:
                     body = [json.dumps({'error_message': '\n'.join(app_iter)})]
                 state['headers'].append(('Content-Type', 'application/json'))
-            state['headers'].append(('Content-Length', len(body[0])))
+            state['headers'].append(('Content-Length', str(len(body[0]))))
         else:
             body = app_iter
         return body