wsgi: Call start_response from a single place.

Prepares for CORS headers auto-generation.

caucase/wsgi.py

@@ -36,7 +36,9 @@ class ApplicationError(Exception):
   WSGI HTTP error base class.
   """
   status = _getStatus(httplib.INTERNAL_SERVER_ERROR)
-  response_headers = ()
+  @property
+  def response_headers(self):
+    return []
 
 class BadRequest(ApplicationError):
   """
@@ -160,15 +162,13 @@ class Application(object):
         traceback.print_exc(file=sys.stderr)
         raise ApplicationError
     except ApplicationError, e:
-      start_response(
-        e.status,
-        list(e.response_headers),
-      )
+      status = e.status
+      header_list = e.response_headers
       result = [str(x) for x in e.args]
-    else:
     start_response(
       status,
-        header_list,
+      header_list + [
+      ],
     )
     return result