test(encoding/form): well konw types test (#2147)

[shengzhou1216]

Description (what this PR does / why we need it):

add tests for encoding/form/well_konw_types.go

Which issue(s) this PR fixes (resolves / be part of):

part of #2147

Other special notes for the reviewers:

[codecov-commenter]

Codecov Report

Merging #2160 (d8d583f) into main (b6954d1) will not change coverage.
The diff coverage is n/a.

❗ Current head d8d583f differs from pull request most recent head 49aaa34. Consider uploading reports for the commit 49aaa34 to get more accurate results

@@           Coverage Diff           @@
##             main    #2160   +/-   ##
=======================================
  Coverage   79.08%   79.08%           
=======================================
  Files          84       84           
  Lines        3696     3696           
=======================================
  Hits         2923     2923           
  Misses        560      560           
  Partials      213      213           

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