Add .observable() test

test/observable.js

@@ -0,0 +1,25 @@
+import { createClient } from '../';
+import { assert } from 'chai';
+
+
+describe('.observable()', function () {
+  it('should return an Observable', function () {
+    const client = createClient();
+
+    const marko = 'marko';
+    const query = `g.V().has('name', '${marko}').valueMap()`;
+
+    const results$ = client.observable(query);
+
+    let vertex;
+    let error;
+    results$.subscribe(
+      (result) => vertex = result,
+      (err) => error = err,
+      () => {
+        assert.isUndefined(error);
+        assert.deepPropertyVal(vertex, 'name[0]', marko);
+      }
+    )
+  });
+});