should not be able to instantiate an interface (using new)

... for what-ever reason `Interface.new` generates an anonymous class, currently :

```
kares@sputnik:~/workspace/oss/jruby-rack$ irb
jruby-9.0.0.0 :001 > java.lang.Runnable.new
 => #<#<Class:0x79714e09>:0x7dba6f28> 
jruby-9.0.0.0 :002 > java.lang.Runnable.new.class
 => #<Class:0x79714e09> 
jruby-9.0.0.0 :003 > java.lang.Runnable.new.class.name
 => nil 
```
