Feature request: Constraint support emulation

[palashgo]

Is your feature request related to a problem? Please describe. The pollyfilled getUserMedia does not mimic how different browser support different constraint configuration.

Describe the solution you'd like The library should mimic the constraint behaviour of Chrome, Safari and Firefox and fail in cases where real browsers fail as well. A "reverse" adapter-js like behaviour

https://github.com/webrtcHacks/adapter