Fix KeyError while generating a WSGI response

The KeyError exception raised while generating a WSGI response based on
the 413 exception. Because there may be no "Retry-After" attribute in the
exception headers.

Change-Id: I89eb1878acc2195775be17fbcba8088f2e00bac1
Closes-Bug: #1259867
Merge "Fix KeyError while generating a WSGI response"