Parse headers via regexp

[dhoko]

Hey,

I've found this issue using the lib:

console.js:35 Error: parseHeaderValue() failed for image/png; filename="Fleshing out a sketch of a bird for a friend! - ;.png"; name="Fleshing out a sketch of a bird for a friend! - ;.png"
    at u (parse.js:214)

The parsing of headers was done via ; and as it's inside the name here it creates a bug.
So instead of using this ; I replaced it via something from REGEXP_PARAM.
A test will cover this use case now :)