Add cross-frame compatibility to knockout.mapping

[traviskaufman]

I was using knockout.mapping on a project where the plugin needed to be loaded within a sandboxed environment (in this case, an iframe) and noticed that it was in no way behaving as expected. This seemed to be due to two things:

1. exports.getType was looking at an object's constructor to determine what kind of object it was.
2. There was some use of the instanceof operator for type inference.

Because of these, the library would not work unless loaded within the same window context as the script in which it was being used. This PR fixes that issue by using Object.prototype.toString to find the internal [[Class]] property of the object (as per the ECMA spec) and return the proper value from there.

Note that with the current implementation getType will return "object" when passed null. I'm not sure if this is the behavior that you're looking for since in most cases a client doesn't treat a "null" property the same way it treats other objects, but I left it since it's outside the scope of this PR. Happy to add it though if you guys would like! I tried it out and ran the QUnit suite and everything still passed.

p.s. AWESOME knockout plugin! Made my life a whole lot easier for this project!

[traviskaufman]

What's the word on this? Is anyone maintaining this codebase currently? I see PR's open from 4 months ago

[sagacity]

Hi Travis, I currently don't have the time to maintain this project. There has been some discussion going on here for a potential replacement. You are also welcome to fork and extend, of course!

[brigand]

I'm merging this into my fork. instanceof causes more problems than cross-frame stuff. I've read several articles about type checking primitives, and this seems to be the most reliable method, and works on all browsers.

Btw, whitespace fixes are nice, but should be done on a separate commit (I'm guessing your editor/IDE did it automatically).