Capture variables from env for closure should be immutable

[killme2008]

For example

  String exp = "a=1; b = lambda(x) -> a+ x end ; a=4 ; b(5)";
    System.out.println(AviatorEvaluator.execute(exp)); // output 6

should output 6, but now is 9.

[killme2008]

Now it will throw an exception for such assigment:

Exception in thread "main" com.googlecode.aviator.exception.ExpressionRuntimeException: Can't assignment value to captured variable.The `a` is already captured by lambda.
	at com.googlecode.aviator.utils.Env.put(Env.java:217)
	at com.googlecode.aviator.utils.Env.put(Env.java:1)
	at com.googlecode.aviator.runtime.type.AviatorJavaType.setValue(AviatorJavaType.java:264)
	at Script_1547125754122_0/1344622638.execute0(Unknown Source)
	at com.googlecode.aviator.ClassExpression.execute(ClassExpression.java:71)
	at com.googlecode.aviator.AviatorEvaluatorInstance.execute(AviatorEvaluatorInstance.java:741)
	at com.googlecode.aviator.AviatorEvaluator.execute(AviatorEvaluator.java:439)
	at com.googlecode.aviator.AviatorEvaluator.execute(AviatorEvaluator.java:451)
	at com.googlecode.aviator.AviatorEvaluator.execute(AviatorEvaluator.java:472)
	at com.googlecode.aviator.example.LambdaExample.main(LambdaExample.java:12)

[killme2008]

https://github.com/killme2008/aviator/releases/tag/aviator-4.1.2