Fix remove_entity_headers helper function

[hramezani]

Based on the comment on remove_entity_headers helper function:

Removes all the entity headers present in the headers given.
According to RFC 2616 Section 10.3.5,
Content-Location and Expires are allowed as for the
"strong cache validator".

Content-Location and Expires should be allowed.

[codecov-io]

Codecov Report

Merging #1415 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1415   +/-   ##
=======================================
  Coverage   83.45%   83.45%           
=======================================
  Files          17       17           
  Lines        1704     1704           
  Branches      322      322           
=======================================
  Hits         1422     1422           
  Misses        217      217           
  Partials       65       65

Impacted Files	Coverage Δ
sanic/helpers.py	100% <ø> (ø)	⬆️

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