Differ GET from POST in @@daily-issue

GET is safe while POST can do real stuff.

HTTP Responses:

===================  ============  =====  ============
Method/Precondition  Not yet sent  Empty  Already Sent
===================  ============  =====  ============
GET                     100         204       200
POST                    200[*]_     204       409
===================  ============  =====  ============

.. [*] send issue first.
Include documentation for @@daily-issue