__isProxy property should not be enumerable (IE11 issue)

[ElliotNB]

In IE11, __isProxy is enumerable when iterating over an object's properties like this:

for (var requiredField in this.data.missing_fields) {

It should not be enumerable.

[Rezyan]

@ElliotNB I will be working on this issue soon.

[ElliotNB]

@BrokenSourceCode Sounds great, thanks for the heads up! For this particular issue, I believe it was only impacting IE11 so for testing that'll require IE11, the proxy.min.js polyfill included, etc. If it's possible to create a unit test that replicates the error and to demonstrate that the fix is working afterwards, that would also be fantastic. With this library I try to be as TDD as possible 8)

[Rezyan]

@ElliotNB If this issue is only on IE11, I invite you to read my message here: https://github.com/ElliotNB/observable-slim/issues/6