Fix NullPointerException when equals or hashCode are called on proxy instance

They look something like this:

java.lang.NullPointerException
	at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:88)
	at feign.$Proxy16.equals(Unknown Source)

In  my particular instance, I had a proxy created by Feign registered in a Spring application context,
and it resulted in a NullPointerException on application shutdown.
Merge pull request #61 from davidmc24/fix-equals-hashcode

Fix NullPointerException when equals or hashCode are called on proxy instance