TINKERPOP-2735: Backport fix to close request error in UnifiedChannelizer to 3.5-dev

[xiazcy]

A fix to the error message on session close reported in https://issues.apache.org/jira/browse/TINKERPOP-2735.

Session close request will output Unrecognized content error message in 3.5.x server, the fix is in 3.6.0 where UnifiedHandler has an added check to handle close requests, whereas the 3.5.x server does not, and that causes the request to fall through into the exception here with a null content type for the gremlin args field.

This PR backports that check in 3.6 for close requests into 3.5-dev

[codecov-commenter]

Codecov Report

Merging #1705 (ae720be) into 3.5-dev (287499e) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           3.5-dev    #1705      +/-   ##
===========================================
- Coverage    63.35%   63.27%   -0.09%     
===========================================
  Files           23       23              
  Lines         3553     3553              
===========================================
- Hits          2251     2248       -3     
- Misses        1129     1131       +2     
- Partials       173      174       +1     

Impacted Files	Coverage Δ
gremlin-go/driver/client.go	79.12% <0.00%> (-3.30%)	⬇️

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