Hold every write to POST and refuse PUT, PATCH and DELETE #73

Parent: #72. Blocked by #42.

Goal

TestNonConformingRoutes holds the whole method rule, not only its read half.

Acceptance criteria

• A route whose method is neither GET nor POST is reported as non-conforming.
• A non-read RPC served over GET is reported as non-conforming.
• The exception set is still empty.

Verification

make test