[JENKINS-25118] Allow signatures with primitives

[JENKINS-25118] (https://issues.jenkins-ci.org/browse/JENKINS-25118)

As `Class#isinstance(Object)` always returns `false` for primitives, the sandbox fails to identify methods with primitive arguments.
http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#isInstance%28java.lang.Object%29

This change allows the sandbox work for methods with primitive arguments.