[Bugfix] Remove the last EOS token unless explicitly specified

[jsato8094]

This fixes the unintended exposure of the EOS token. When setting up an OpenAI API-compatible server, the current implementation exposes the EOS token to the user, even though the API user does not need to know what the tokenizer's EOS token is. Removing the EOS token unless explicitly specified solves this problem.

This was the default behavior until v0.3.2, but it appears to have changed between v0.3.2 and v0.4.2.

FIX #4814

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[DarkLight1337]

Thanks for fixing this!

To avoid similar regressions in the future, can you add some unit tests to verify this behaviour?