fix utf16 decode issue

[magodo]

Thank for your awesome effort on this library. I'm using it to parse the ID3v2 tags for some international files, and encounter issues when some frame is encoded with utf16. In which case it doesn't work well.

Finally I figured out there where two issues:

1. when trying to get the frame identifier, it has no need to use the evaluated encoding type, since it always guaranteed to be 4 chars.
2. when decode utf16 code point, what you got as input is not the code point itself. For example, the code point of Chinese character "人" 's codepoint is \xbaN, but what you got as input is: [186, 78]. So you should not directly use String.fromCodePoints(). Instead use the official lib utf.

[NiKoTron]

Hi, @magodo! Thank you for a founded issue and made a detailed explanation of it. But could you please re-create Pull Request on to the develop branch?