fix: use prefix with etag middleware

[Krukov]

Fix for #218

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 93.76%. Comparing base (c2d7e32) to head (b0364e6).
Report is 2 commits behind head on master.

Files	Patch %	Lines
cashews/contrib/fastapi.py	90.00%	2 Missing and 3 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
- Coverage   93.79%   93.76%   -0.04%     
==========================================
  Files          54       54              
  Lines        3498     3496       -2     
  Branches      604      604              
==========================================
- Hits         3281     3278       -3     
- Misses        108      109       +1     
  Partials      109      109              

Flag	Coverage Δ
unittests	93.64% <90.00%> (-0.04%)	⬇️

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