chore: test Oxf servers

[monsieurtanuki]

What

• The purpose of this PR is to get rid of annoying emails about some specific tests non passing.
• In that particular case, the OxF servers are sometimes funny in their heads, and getProduct (and of course the corresponding URL) fails with a 504 Gateway Time-out.
• Instead of receiving error messages about problems that do not regard our code but the current status of the OxF servers, we ignore the tests when we have 504 exceptions.

Impacted files

• api_not_food_get_product_test.dart: added tests around 504 Gateway Time-out
• open_food_api_client.dart: minor refactoring
• pubspec.yaml: minor package upgrades

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (2c6ba80) 75.80% compared to head (78bd96d) 75.79%.

Files	Patch %	Lines
test/api_not_food_get_product_test.dart	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #861      +/-   ##
==========================================
- Coverage   75.80%   75.79%   -0.02%     
==========================================
  Files         224      224              
  Lines        7903     7902       -1     
==========================================
- Hits         5991     5989       -2     
- Misses       1912     1913       +1     

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[monsieurtanuki]

Closing as stale.