Fix non-numeric exclusive Range#minmax bug

The implementation of Range#minmax added in d5c60214c45 causes the
following incorrect behaviour:

  ('a'...'c').minmax => ["a", ["a", "b"]]

instead of

  ('a'...'c').minmax => ["a", "b"]

This is because the C implementation of Range#minmax (range_minmax)
directly delegates to the C implementation of Range#min (range_min) and
Range#max (range_max), without changing the execution context.

Range#max's C implementation (range_max), when given a non-numeric
exclusive range, delegates to super, which is meant to call
Enumerable#max. However, because range_max is called directly by
range_minmax, super calls Enumerable#minmax instead, causing the
incorrect nesting.

Perhaps it is possible to change the execution context in an optimized
manner, but the simplest solution seems to be to just explicitly
delegate from Range#minmax to Range#min and Range#max.

range.c

@@ -1266,7 +1266,10 @@ range_minmax(VALUE range)
     if (rb_block_given_p()) {
         return rb_call_super(0, NULL);
     }
-    return rb_assoc_new(range_min(0, NULL, range), range_max(0, NULL, range));
+    return rb_assoc_new(
+        rb_funcall(range, rb_intern("min"), 0),
+        rb_funcall(range, rb_intern("max"), 0)
+    );
 }
 
 int

test/ruby/test_range.rb

@@ -146,6 +146,9 @@ class TestRange < Test::Unit::TestCase
     assert_equal([nil, nil], (0...0).minmax)
 
     assert_equal([2, 1], (1..2).minmax{|a, b| b <=> a})
+
+    assert_equal(['a', 'c'], ('a'..'c').minmax)
+    assert_equal(['a', 'b'], ('a'...'c').minmax)
   end
 
   def test_initialize_twice