Fix too lax application/json prefix-only matching

[scop]

What do these changes do?

Fix too lax application/json matching.

For example, application/jsonfoo or application/json-seq should not be treated as valid for JSON.

Are there changes in behavior for the user?

Related issue number

Fixes #5896

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
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  • Please keep alphabetical order, the file is sorted by names.
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  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
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    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
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    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov]

Codecov Report

Merging #6180 (7bc72c7) into master (c408ca1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 7bc72c7 differs from pull request most recent head 434c6c5. Consider uploading reports for the commit 434c6c5 to get more accurate results

@@           Coverage Diff           @@
##           master    #6180   +/-   ##
=======================================
  Coverage   93.29%   93.30%           
=======================================
  Files         102      103    +1     
  Lines       30351    30362   +11     
  Branches     2729     2729           
=======================================
+ Hits        28315    28328   +13     
+ Misses       1858     1857    -1     
+ Partials      178      177    -1     

Flag	Coverage Δ
unit	93.22% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	98.02% <100.00%> (-0.01%)	⬇️
aiohttp/helpers.py	96.92% <100.00%> (ø)
aiohttp/web_request.py	95.90% <100.00%> (-0.01%)	⬇️
tests/test_helpers.py	98.89% <100.00%> (+<0.01%)	⬆️
tests/conftest.py	66.66% <0.00%> (-1.71%)	⬇️
aiohttp/__init__.py	100.00% <0.00%> (ø)
tests/test___all__.py	100.00% <0.00%> (ø)
tests/test_worker.py	22.44% <0.00%> (+2.04%)	⬆️

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[scop]

Note that this will conflict with #6181, will fix the other after one of these is merged.

[asvetlov]

#6181 is merged, please fix conflicts

[scop]

Done/rebased.

[asvetlov]

Would you please replace resp.headers['Content-Type'] check with resp.content_type?
It can simplify things by dropping parameters support (e.g. charset) from regexp.

[scop]

Done.

[webknjaz]

I don't have any additional comments beyond those few nitpicks so I'll let @asvetlov decide when to merge this.

[asvetlov]

LGTM