Fixed subscription, added a subscription test

Subscription is async and therefore must use cast, forgot about that
in the interface. Also if there were not parameters encoding wasn't
working right, so that's also fixed now

lib/changes_communicator.ex

@@ -48,9 +48,8 @@ defmodule CouchDBEx.Worker.ChangesCommunicator do
         selector: pre_final_opts[:selector]
       }
       |> Enum.filter(fn {_, v} -> not is_nil(v) end)
-      |> (fn m -> if(Enum.empty?(m), do: "", else: m) end).()
       |> Enum.into(%{})
-      |> Poison.encode!
+    maybe_body = if Enum.empty?(maybe_body), do: "", else: Poison.encode!(maybe_body)
 
     final_opts =
       pre_final_opts

lib/couchdb_ex.ex

@@ -375,14 +375,14 @@ defmodule CouchDBEx do
                 trying to change it **WILL RAISE a RuntimeError**
   """
   def changes_sub(db, modname, watcher_name, opts \\ []) do
-    GenServer.call(CouchDBEx.Worker, {:changes_sub, db, modname, watcher_name, opts})
+    GenServer.cast(CouchDBEx.Worker, {:changes_sub, db, modname, watcher_name, opts})
   end
 
   @doc """
   Unsubscribe the `modname` watcher from changes in the database.
 
   `modname` is the module's name as known to the supervisor, not the actual running module.
   """
-  def changes_unsub(modname), do: GenServer.call(CouchDBEx.Worker, {:changes_unsub, modname})
+  def changes_unsub(modname), do: GenServer.cast(CouchDBEx.Worker, {:changes_unsub, modname})
 
 end

test/couchdb_ex_test.exs

@@ -154,4 +154,47 @@ defmodule CouchDBExTest do
     )
   end
 
+  test "subscribing works" do
+
+    defmodule ChangesTest do
+      use GenServer
+
+      def start_link(opts) do
+        GenServer.start_link(__MODULE__, nil, opts)
+      end
+
+      def init(_) do
+        {:ok, nil}
+      end
+
+      def handle_info({:couchdb_change, msg}, _) do
+        {:noreply, msg}
+      end
+
+      def handle_call(:get, _from, state) do
+        {:reply, state, state}
+      end
+
+    end
+
+    seed = Integer.to_string(ExUnit.configuration[:seed])
+
+    CouchDBEx.changes_sub("couchdb-ex-test", ChangesTest, ChangesTest)
+
+    CouchDBEx.document_insert_one(%{data: seed}, "couchdb-ex-test")
+
+    # Need to wait a little, it never spawns immidietly
+    Process.sleep(100)
+
+    assert Enum.count(Supervisor.which_children(CouchDBEx.Worker.ChangesCommunicator.Supervisor)) == 1
+
+    %{"doc" => %{"data" => ^seed}} = GenServer.call(ChangesTest, :get)
+
+    CouchDBEx.changes_unsub(ChangesTest)
+
+    # Just to be safe
+    Process.sleep(100)
+
+    assert Enum.count(Supervisor.which_children(CouchDBEx.Worker.ChangesCommunicator.Supervisor)) == 0
+  end
 end