Addressing GRECLIPSE-1708

Since Groovy allows it, this code change adjusts `ASTTransformationCollectorCodeVisitor` to also allow the `classes` attribute to be just a single class (or `ClassNode`, specifically). This is needed to address the issue reported in GRECLIPSE-1708, which has the symptom:

    java.lang.RuntimeException: nyi implemented in eclipse: need to support: org.codehaus.groovy.ast.expr.ClassExpression@4fc15e5[type: transform.PropertyEnhancementTransformation] (class=class org.codehaus.groovy.ast.expr.ClassExpression)