Fix runtime error when testing equality of Json.Encode.null values

[ThomasWeiser]

Function eq in Utils.js is currently only suitable for normal Elm types, not for arbitrary (speak Json.Encode) values.

As a result, comparing Json.Encode.nullwith anything non-null throws a runtime error.

Example:

Json.Encode.null == Json.Encode.object []

This PR fixes these cases.

Unit-tests are added to tests/Test/Equality.elm

[ThomasWeiser]

Rebased to current master.
Ready for being merged for upcoming relase 3.0.0

[ThomasWeiser]

Rebased to current master.
Ready for being merged for upcoming relase 3.0.0

[evancz]

6f5fbb8 does a check like this, and 1c00777 makes a note that it will not be possible to check for equality on Json.Encode.Value once a proper fix is implemented. That scenario is fundamentally problematic because a Json.Encode.Value can come through ports, meaning it can have functions.

This issue blocked because it was not clear what the ultimate fix would be. Now that it is clearer, I am fine with doing the stopgap.

[evancz]

6f5fbb8 does a check like this, and 1c00777 makes a note that it will not be possible to check for equality on Json.Encode.Value once a proper fix is implemented. That scenario is fundamentally problematic because a Json.Encode.Value can come through ports, meaning it can have functions.

This issue blocked because it was not clear what the ultimate fix would be. Now that it is clearer, I am fine with doing the stopgap.