GString doesn't call correct toString method when it's been changed via the metaClass mechanism

This is best shown by a small example. I've attached a trivial patch as well.

class A {
String toString()

}

A.metaClass.toString = { ->
"changed";
}

def a = new A();
println a.toString();
println "$

";

brandon@Brandon:~/localtmp/groovy-1.5.4$ unset GROOVY_HOME
brandon@Brandon:~/localtmp/groovy-1.5.4$ groovy-unpatched test.groovy
changed
unchanged
brandon@Brandon:~/localtmp/groovy-1.5.4$ groovy test.groovy
changed
changed