Consider dropping lowercasing requirement for the type attribute

[wisniewskit]

While working on Gecko's Fetch compliance, I noted that there are web platform tests which do not expect blob types to be lowercased. @annevk suggested that dropping the lowercasing requirement might be an option, so I'm filing a bug to verify that option.

[annevk]

To elaborate a tiny bit, parse a MIME type already lowercases the type, subtype, and parameter names, but lowercasing parameter values technically makes us lose information, so shouldn't be done.

[annevk]

This will probably fall out of #123.

[annevk]

Let's duplicate this into #43.