Add sdp params to step-5&6 to improve compatibility

[platy]

I've implemented @nitobuendia 's suggestions on #33.

I don't know the purpose of the 2 extra fields but it seems they are only needed when there is more than one Ice candidate - which seems to be more frequent on firefox than chrome when testing locally

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[platy]

I signed it!

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[platy]

Hey, I've just seen this is not merged - I'm not working with webrtc any more and can't remember what this is but can we merge it or close it?