Fix header processing

[munrojm]

This PR fixes a variable name issue that was overwriting the temporary Response object FastAPI uses to extract header data to use in the response. This did not allow the correct header to be processed by a given Resource class.

[codecov]

Codecov Report

Merging #679 (0753c8d) into main (b506eb8) will not change coverage.
The diff coverage is 55.55%.

@@           Coverage Diff           @@
##             main     #679   +/-   ##
=======================================
  Coverage   89.43%   89.43%           
=======================================
  Files          42       42           
  Lines        3048     3048           
=======================================
  Hits         2726     2726           
  Misses        322      322           

Impacted Files	Coverage Δ
src/maggma/api/resource/s3_url.py	59.18% <0.00%> (ø)
src/maggma/api/resource/read_resource.py	95.29% <71.42%> (ø)

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