Fixed bug concerning Enum's method getDeclaringClass()

Looks like I had found a bug.
When we try copy enums, modelmapper thinks that getDeclaringClass() in java.util.Enum is property getter, and iterates through whole Class graph, which slows down ModelMapper's initialization. So, I think it would be nice, if we hide all Enum's methods, though it is discussable.
Using this fix modelmapper's I made initialization twice as faster, but actually, modelmapper's initialization still not that good for unit tests.