[DO NOT MERGE] Add 2.7 Range#minmax monkeypatch

Unfortunately, the 2.7 implementation actually contains a bug which
causes the underlying C implementation of Range#max to delegate to
Enumerable#minmax instead of Enumerable#max, since the optimized
Range#minmax C implementation skips the Ruby call to Range#max, and
calls straight into the C function range_max, thereby failing to change
the execution context, and causing the incorrect super call.

Pending a fix in MRI, Range#minmax is monkeypatched with an equivalent
fix, allowing the specs to be worked on. Once the fix is merged
upstream, this commit can be reverted to remove the monkeypatch, and the
specs should continue to pass.

core/range/minmax_spec.rb

@@ -11,6 +11,17 @@
     @y.should_receive(:<=>).with(@y).any_number_of_times.and_return(0)
   end
 
+  ruby_version_is '2.7' do # TODO: Remove this before merging!
+    class Range
+      # This monkey patch roughly matches the implementation of the bug fix in MRI's range.c
+      def minmax
+        return super if block_given?
+
+        [min, max]
+      end
+    end
+  end
+
   describe 'by enumerating the range' do
     before(:each) do
       @x.should_receive(:succ).once.and_return(@y)