Fix get type

[woylie]

resolves #63

To cover all cases, I had to add channel2 and channel3 fields to the reminder schema.

• channel: identity_by_fields and type_field
• channel2: none of these options
• channel3: only type_field

If the field passed to the function is an array field, the function will return nil, since the type can only be determined by looking at a single item.

I found another related issue while looking into this, I'll open another PR for that.

I added dialyzer and excoveralls to the project, I hope you don't mind.

[woylie]

Please merge #71 first.

[woylie]

@mathieuprog This is ready for review. I split the workflow to run the tests in the matrix, but the remaining steps only with the latest Elixir version. The CI job from the last PR failed because the formatter did different things in different versions.

[mathieuprog]

I'll review this as soon as I have some alone-time!

[SteffenDE]

This also fixes an issue I'm seeing where get_polymorphic_type returns nil when a map is passed (happens in a nested form).

[woylie]

@mathieuprog I don't want to rush you, but is there a chance to get this merged soon?

[mathieuprog]

Sorry I'll get to it in coming days

[AHBruns]

Just wanted to give this a little bump. I'm currently affected by this bug. Please let me know if there's anything I can do to help this PR along.

[sodapopcan]

Also here to bump.