Remove $key and $value on calls to set

[jteplitz]

We're adding these properties to objects returned from firebase, but if someone modifies these objects and then calls set the set call will fail because these are invalid properties in a firebase object.
I think we should automatically remove these properties if they're present in a set call since they aren't valid firebase keys anyways.

[doczoidberg]

I wondered if I am doing something wrong because of this.

[pfried]

related to #190 and #197

[jeffbcross]

Interesting, I believe the Firebase SDK behavior prior to 3 was to just ignore the properties.

[jteplitz]

@jeffbcross It was. It looks like the new behavior is to throw an error whenever a key has an invalid character.

[katowulf]

To be technically correct, the old behavior was to ignore any properties that begin with $ or _. Other invalid keys (for example, a key containing a #, ., $, et al) would still throw an error.

[Fabrik19SH]

Hi, i think you should also remove the $exists too.