fix wsproxy keepHost bug

[cyrnicolase]

#1166

[suchen-sci]

Hi, can you also help update this doc https://github.com/megaease/easegress/blob/main/docs/02.Tutorials/2.6.Websocket.md and https://github.com/megaease/easegress/blob/main/docs/07.Reference/7.02.Filters.md#websocketproxy for future users to avoid this problem? Maybe add a simple example to it?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4f0842a) 81.01% compared to head (88751bb) 81.03%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1168      +/-   ##
==========================================
+ Coverage   81.01%   81.03%   +0.02%     
==========================================
  Files         149      149              
  Lines       16962    16967       +5     
==========================================
+ Hits        13741    13749       +8     
+ Misses       2564     2562       -2     
+ Partials      657      656       -1     

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