fix(Http.spec.js): don't chain Map.prototype.set()

While Map.prototype.set() is meant to be chainable, in Spidermonkey and V8,
these methods currently explicitly return undefined.

While there have been bugs reported and patches submitted to fix these two
engines, it's a good idea to remove chaining in the short-term.

This will get tests passing in Firefox once the next release of Traceur is pulled in.