Allow to parse a list with quality values formatted as strings
per RFC 7231, section 5.3.1.

e.g. "a,b;q=0.1,c;q=0.4" is parsed as a list of WeightedValue:

• (a, 1.0)
• (b, 0.1)
• (c, 0.4)

This is the format used for the ACCEPT_LANGUAGE HTTP header.

Unit tests provided