use Common.extend() in Constraint

[SillyFreak]

this should allow specifying plugin among others in Constraint.create().

See #817.

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Sadly I do have basically two questions. First, running npm install and then npm run test gave me errors about missing poly-decomp so I added that. Is that a missing dependency? The demos seemed to run fine without it.

Second: I tried to add in a dummy extend call, like this:

var defaults = {
};

var constraint = Common.extend(defaults, options);

This leads to a stack overflow, as the options (maybe bodyA or bodyB?) seem to be recursive. Using Common.extend(defaults, false, options); works, but I'm not sure what the ultimate consequences of that would be. I presume not different from the current var constraint = options; so I've gone ahead and used that, but I still wanted to ask your opinion on that as it's different than Body's use of extend().