Make JSON media type matching case insensitive

[scop]

What do these changes do?

Per RFC 2045, section 2 and appendix A, make JSON media type matching case insensitive.

Are there changes in behavior for the user?

Related issue number

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
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • 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
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .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."

[scop]

Note that this will conflict with #6180, will fix the other after one of these is merged.

[codecov]

Codecov Report

Merging #6181 (c4a3b9d) into master (9c7f3d3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6181   +/-   ##
=======================================
  Coverage   93.31%   93.31%           
=======================================
  Files         102      102           
  Lines       30325    30329    +4     
  Branches     2724     2724           
=======================================
+ Hits        28299    28303    +4     
  Misses       1849     1849           
  Partials      177      177           

Flag	Coverage Δ
unit	93.24% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
aiohttp/helpers.py	96.92% <100.00%> (ø)
tests/test_helpers.py	98.88% <100.00%> (+<0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9c7f3d3...c4a3b9d. Read the comment docs.

[asvetlov]

LGTM, thanks!