Differentiate between block args with defaults and without.

Fixes rspec/rspec-mocks#714.
Methods and lambdas share semantics in terms of how `return` works and how they handle arguments.  To convert a proc to a lambda on MRI, we need to define a method using the proc, and then, use `Method#to_proc` to get back proc that's a lambda proc.

I just chose `to_lambda` as the name for the method but it doesn't really matter what it's named.