Request: Additional case-normalizing before passing into mime.ParseMediaType

[CyrusRoshan]

I currently have an email header value of text/html; charset=utf-8; CHARSET=UTF-8 (in an email from Air Canada, if you were wondering), similar to the text/html; charset=utf-8; charset=UTF-8 reported here.

I think lowercasing the ctype value in mediatype.Parse before passing into mime.ParseMediaType would fix this case with the the intended behavior.