#206 - Set class loader for dummy method proxies.

The Enhancer used to create the proxy instances wasn't explicitly
setting the class loader to use, which meant by default the proxied
class's class loader was chosen. This is fine for controller classes, but
in some containers the return types for the controller may have a
class loader that doesn't have access to Spring, resulting in
ClassNotFoundExceptions. For example, returning a JAX-RS class JBoss.
Using the controller's class loader when proxying the return type solves
this. 

Related issues: #136.