Use audio codec for existing audiofile

[zalgo3]

moviepy.video.VideoClip.write_videofile ignores audio_codec when audio is a file name. I have fixed it in this PR.

Although #1906 was closed when v2 was released, I am reopening this PR because I still believe it will improve functionality.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
  • moviepy.video.VideoClip.write_videofile("output.mp4", audio="input.wav", fps=60, audio_codec="aac") have run ffmpeg with -acodec copy. With this PR, it will run the command with -acodec aac.
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

[OsaAjani]

That seems like a nice and coherent addition. Thanks for your contribution !