Make OPAssign work with getter/setter

Current rock generates errors on the following code:

	foo: class{
		init: func(){}
		a: Int = 0
		b: Int{
			get { a }
			set(c){ a = c }
		}
	}

	bar := foo new()
	bar b += 1

When getter/setter appear with Operator_Assign, the getter in right side can not be resolved correctly. 
This commit fix this bug by unwrapping and resolving getter in unwrapAssign function.