Added test_metadata

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.
Fixed compile error in test_security_null
Problem: GSSAPI server doesn't parse ZAP user id

Solution: add this to ZAP reply processing.
Fixed man page example
ZAP vs. ZMTP metadata priority fixed
Merge pull request #1003 from hintjens/master

Added test_metadata