Fix message send flags

[Dorukyum]

Summary

Closes #1722 by making sure that flags is always an integer and not an instance of flag_value.

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.

[codecov]

Codecov Report

Merging #1723 (3d8cf3d) into master (92de03a) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1723   +/-   ##
=======================================
  Coverage   33.19%   33.19%           
=======================================
  Files          96       96           
  Lines       18414    18414           
=======================================
  Hits         6113     6113           
  Misses      12301    12301           

Flag	Coverage Δ
pytest	33.19% <0.00%> (ø)

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

Impacted Files	Coverage Δ
discord/abc.py	22.82% <0.00%> (ø)

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