Handle Stream and PipeReader content types correctly

[martincostello]

• Do not generate schema models for Stream and PipeReader.
• Only allocate the array once, rather than on every call.
• Fix warning by removing unused parameter from private method.

Relates to item 3 in https://github.com/dotnet/aspnetcore/issues/44677.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.65%. Comparing base (a772c94) to head (044a995).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2784      +/-   ##
==========================================
+ Coverage   91.62%   91.65%   +0.02%     
==========================================
  Files          91       91              
  Lines        2998     3007       +9     
  Branches      514      514              
==========================================
+ Hits         2747     2756       +9     
  Misses        251      251              

Flag	Coverage Δ
Linux	91.65% <100.00%> (+0.02%)	:arrow_up:
Windows	91.65% <100.00%> (+0.02%)	:arrow_up:
macOS	91.65% <100.00%> (+0.02%)	:arrow_up:

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