Add Ecto.Changeset pattern match, fix data sent to mutate().

[martinthenth]

The Ecto.Changeset, valid?: false pattern match allows people to use Ecto Changeset in Dlex like they would with Ecto. If the changeset is valid, data is mutated in Dgraph, else returns {:error, changeset}.

For example: changeset = Post.create_changeset(Post{}, params) with Repo.set(changeset)

Furthermore, inserting data via changesets resulted in error: no function clause matching in MyApp.Post.__schema__/2. It also added unwanted fields from Ecto Changeset like __meta__, id, and predicates from schema with nil values. This was because data = struct(type, Map.put(changes, :uid, uid)) created a struct over which encode_kv() couldn't pattern match.

Now, data is inserted as expected using the Ecto.Changeset changes field.

[emhagman]

@liveforeverx This looks good to me, just wanted you to double check

[emhagman]

@martinthenth Do you mind adding the two tests for the examples you used (valid versus invalid changeset) here https://github.com/liveforeverx/dlex/blob/master/test/dlex/repo_test.exs so we make sure we don't break this going forward. We currently only have a test for using Repo.set on a new model struct

[martinthenth]

Sure, I added two tests for Repo.set() with valid and invalid Ecto Changeset.

[liveforeverx]

Thanks, @martinthenth ! 💛 💚 💙

Thanks @emhagman for review!