Get accepted languages from HTTP request
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.
Test Plan: Unit tests provided
Reviewed By: dereckson
Differential Revision: https://devcentral.nasqueron.org/D1634