Python exception when calling AudioSegment.export() with bitrate of int type (Issue #784)

[rwmnau]

This pull request is associated with issue https://github.com/jiaaro/pydub/issues/784.

The fix checks if the bitrate passed into the method is an int, and if it is, it recasts it as a string before adding it to the args list. If it's not present or any other type, it leaves it alone.