Log Message:
Update to 6.18

Upstream changes:
6.18      2018-06-05 16:29:15Z
    - Revert status_message to original code (GH#111) (Theo van Hoesel)

6.17      2018-06-05 01:55:34Z
    - Documented status code 451 in the list of constants (GH #104) (Robert \ 
Rothenberg)
    - Status code 451 is cachable by default, as per RFC 7725 (GH #104) (Robert \ 
Rothenberg)
    - Add default status_message for unknown status codes (GH#105) (Robert \ 
Rothenberg)
    - Corrections to the documentation (GH#102) (Jonas B. Nielsen)