Merge branch '1.0' into 1.1
Check quality > 0.0 in FormatNegotiator::getBestFormat()

The getBestFormat() method returns a format or null. When quality == 0.0,
then it should return null rather than the negotiated format.

Fix #29

See: https://github.com/willdurand/Negotiation/issues/29
Merge branch '1.0' into 1.1
Merge branch '1.1' into 1.2
Merge branch '1.2'