fix: on xpect asserts over a variable look for the tvar for the varia…

…ble instead of the reference.

This is specifically necessary when the variable ref is the LHS of an assignment.

org.uqbar.project.wollok.tests/src/org/uqbar/project/wollok/tests/typesystem/xpect/dates.wlk.xt

@@ -5,7 +5,7 @@ object dateTests {
 	// XPECT methodType at m1 --> () => Date
 	method m1() = new Date()
 
-	// X PECT methodType at m2 --> (Number) => Date
+	// XPECT! methodType at m2 --> (Number) => Date
 	// method m2(day) = new Date(day, day, 2018)
 
 	// XPECT methodType at m3 --> (Number) => Date