fix MQTTProxy test case random error

[suchen-sci]

replace nil mapper to mock mapper in test TestHTTPTransferHeaderCopy to prevent test random fail.

[codecov-commenter]

Codecov Report

Merging #752 (933a335) into main (870b7ff) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #752   +/-   ##
=======================================
  Coverage   80.49%   80.49%           
=======================================
  Files          97       97           
  Lines       10785    10785           
=======================================
  Hits         8681     8681           
  Misses       1619     1619           
  Partials      485      485           

Impacted Files	Coverage Δ
pkg/protocols/httpprot/request.go	80.32% <0.00%> (ø)

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