Fix - more robust param coercion for MediaDeviceConstraints

[b-ma]

Fix #193

[orottier]

Before you release. I am planning a release for the base lib this weekend. Might want to include it, mostly performance and bug fixes, no API changes

[b-ma]

ok thanks!

[b-ma]

For information I plan to have a look at these points while I'm on this

• clamp input to 32 channels (cf. Mic crashes when number of channels > 32 #159)
• report the MediaStreamTrack number of channels
  "The number of channels of the output corresponds to the number of channels of the MediaStreamTrack."
  https://webaudio.github.io/web-audio-api/#MediaStreamAudioSourceNode
  https://webaudio.github.io/web-audio-api/#MediaStreamTrackAudioSourceNode

Hopefully I'll be able to propose something tomorrow that we can include in the release too (otherwise it will probably have to wait for September :)