If pedantic_wpt is set, reject invalid header names instead of passing them through.

We currently give a warning for invalid or problematic characters in header names:
- This string contains 8-bit characters in the range 0x80 - 0xFF.
- Per the Fetch specification, the Headers class may only accept header names and values which contain 8-bit characters. That is, they must not contain any Unicode code points greater than 0xFF.

In `pedantic_wpt` mode we have to throw on these instead. However this could be really annoying for anyone implementing a proxy, who is just trying to pass through what the client gave them. We might need to create a `very_pedantic_wpt` flag too...