Added test for the case of objects as cells

I encountered this in Drupal 8, where cells implemented the
MarkupInterface. It was objects that, when cast to a string,
returned the translated string.

Before 5ca540daa7, this new test
would fail.

tests/methods/ObjectThatHasToStringMethod.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace ParseCsv\tests\methods;
+
+/**
+ * Class HasToString is just a helper to test if cells can be objects.
+ */
+class ObjectThatHasToStringMethod {
+
+    public function __toString() {
+        return 'some value';
+    }
+}