Saving ID3 tags marked as case-insensitive causes duplicated TXXX frames

If you save e.g. explicitly with `$set(Replaygain_Album_Gain,foo)` and there is an existing TXXX:REPLAYGAIN_ALBUM_GAIN frame saving the file will result in both TXXX:REPLAYGAIN_ALBUM_GAIN and TXXX:Replaygain_Album_Gain

But since replaygain_album_gain is marked as being treted case insensitive the expected result would be to have only a single TXXX:REPLAYGAIN_ALBUM_GAIN with the new value "foo" after saving.