return_handler: support return values of ({string, []byte}, error)

[unknwon]

Describe the pull request

Both (string, error) and ([]byte, error) are reasonable pairs of return values.

Link to the issue: #117

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

Followups

• Update docs to include all possible combinations of return values

[codecov]

Codecov Report

Merging #118 (1f803c7) into main (4dcd516) will increase coverage by 0.07%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   87.66%   87.74%   +0.07%     
==========================================
  Files          17       17              
  Lines        1135     1142       +7     
==========================================
+ Hits          995     1002       +7     
  Misses        101      101              
  Partials       39       39