[PR #6305/7f325135 backport][3.9] Made chunked encoding HTTP header check stricter

[patchback]

This is a backport of PR #6305 as merged into master (7f32513).

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[codecov]

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@@          Coverage Diff           @@
##             3.9    #6307   +/-   ##
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  Coverage       ?   93.30%           
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  Files          ?      103           
  Lines          ?    30366           
  Branches       ?     2730           
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  Hits           ?    28333           
  Misses         ?     1857           
  Partials       ?      176           

Flag	Coverage Δ
unit	93.22% <0.00%> (?)

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