Fix enum arg handling in Enumerator Nexter

This fixes #264.  I noticed that issue had the same symptoms that #807 addressed for `each_with_index`.  This pull extends the #807 fix to cover the `next` case.

Note also that the regression test `no_args_method.new.enum_for(:my_method).next.should == nil` added here actually hangs prior to this fix (the nexter queue blocked forever waiting for a value that's not coming; now we explicitly feed it the `nil`), so avoiding that is a nice bonus.

Between the updated arg handing and avoiding that hang, it seems like there's a chance some specs can be untagged now... I'll have a look soon.