Remove user input from Bad Request responses

[RedRoserade]

As discussed via e-mail, this PR removes user-input from the HTTP Bad Request responses that result from invalid transports or session IDs.

All unit tests seem to pass locally.

[codecov-commenter]

Codecov Report

Merging #250 (b84be34) into main (c2603e9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #250   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         1894      1894           
  Branches       343       343           
=========================================
  Hits          1894      1894           

Impacted Files	Coverage Δ
src/engineio/server.py	100.00% <100.00%> (ø)

---

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 c2603e9...b84be34. Read the comment docs.

[miguelgrinberg]

Thank you so much!