Implement Method#curry, which simply delegates to Proc#curry.

Also fixes an existing bug in JRuby which would ignore the argument
passed to Proc#curry() and would instead always use the Proc's arity.
With that fixed, Proc#curry properly works with varargs now.

The fixed regression was introduced in 72703450108175e45fb4b21fb7d07676e001c952

Conflicts:
	core/src/main/java/org/jruby/RubyMethod.java
Merge pull request #2133 from cheald/method_curry

Implement Method#curry, which simply delegates to Proc#curry.