FIX: ViewableData::obj() would sometimes return an empty object

For instance, this happens when these criteria are met:
  1) No casting has been specified for a method in $casting.
  2) A template accesses the field without any casting
  3) Any casts by the template will now yield an empty object.

After a brief look at the commit history, it can seem like this bug is several years old, unless it is a side-effect of other changes in the code.

== Steps to reproduce ==
Add two methods to be accessed by a template. Make sure you do not define an entry in $casting for them:
	public function Testus() {
		return "Te<x>t1";
	}

	public function Testus2() {
		return "Te<x>t2";
	}

Add this to a template:
	<p>
		First access:<br />
		"$Testus" : "$Testus.XML"<br />
		"$Testus2.XML" : "$Testus2"<br />
	</p>
	<p>
		Second access:<br />
		"$Testus" : "$Testus.XML"<br />
		"$Testus2.XML" : "$Testus2"<br />
	</p>

Open the page in a browser, and you will get:
 First access:
"Tet1" : ""
"Te<x>t2" : "Tet2"

Second access:
"Tet1" : ""
"" : "Tet2"

We see that any cast can yield an empty string.
Unit test for bugfix in ViewableData::obj().
TEST: additional test for ViewableData not wrapping cached strings