Fix Range#include? for beginless exclusive string ranges

Previously, include? would return true for the end of the range,
when it should return false because the range is exclusive.

Research and Analysis by Victor Shepelev.

Fixes [Bug #18577]

range.c

@@ -1792,6 +1792,9 @@ range_include_internal(VALUE range, VALUE val, int string_use_cover)
         else if (NIL_P(beg)) {
 	    VALUE r = rb_funcall(val, id_cmp, 1, end);
 	    if (NIL_P(r)) return Qfalse;
+            if (RANGE_EXCL(range)) {
+                return RBOOL(rb_cmpint(r, val, end) < 0);
+            }
             return RBOOL(rb_cmpint(r, val, end) <= 0);
         }
 	else if (NIL_P(end)) {

test/ruby/test_range.rb

@@ -604,6 +604,10 @@ class TestRange < Test::Unit::TestCase
     assert_include(0...10, 5)
     assert_include(5..., 10)
     assert_not_include(5..., 0)
+    assert_include(.."z", "z")
+    assert_not_include(..."z", "z")
+    assert_include(..10, 10)
+    assert_not_include(...10, 10)
   end
 
   def test_cover