Fix Allow header according to RFC 7231

[unbyte]

6.5.5. 405 Method Not Allowed

The 405 (Method Not Allowed) status code indicates that the method received in the request-line is known by the origin server but not supported by the target resource. The origin server MUST generate an Allow header field in a 405 response containing a list of the target resource's currently supported methods.

close #2397

[codecov[bot]]

Codecov Report

Merging #2476 (9af3803) into master (b01605b) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2476   +/-   ##
=======================================
  Coverage   98.64%   98.64%           
=======================================
  Files          41       41           
  Lines        1989     1993    +4     
=======================================
+ Hits         1962     1966    +4     
  Misses         15       15           
  Partials       12       12           

Impacted Files	Coverage Δ
gin.go	99.02% <100.00%> (+0.01%)	:arrow_up:

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[antonjah]

@unbyte @appleboy Any news on this?

[appleboy]

@antonjah conflicts

[Crocmagnon]

I opened a new PR using @unbyte's commits but fixing conflicts, hoping this can get merged & released soon :)

[appleboy]

closed PR and move to https://github.com/gin-gonic/gin/pull/3759