Test failure for data://test:test/,X in the data-url crate

[djc]

There's a panic here:

thread 'data: URL "data://test:test/,X"' panicked at 'Mime { type_: "text", subtype: "plain", parameters: [("charset", "US-ASCII")] }', .tests/wpt.rs:20:9

which appears to have been around at least since the data-url crate was moved into this repository around Feb 2018 (cc670f6).

[djc]

For some reason this does not show up as a test failure, which seems strange. Should investigate.

[djc]

Apparently this might be expected: #473.

[djc]

This was fixed by the changes in #473.