dup String#split return value

* string.c (rb_str_split): return duplicated receiver, when no
  splits.  patched by tompng (tomoya ishida) in [ruby-core:82911],
  and the test case by Seiei Miyagi <hanachin@gmail.com>.
  [Bug#13925] [Fix GH-1705]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60002 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -7371,7 +7371,7 @@ rb_str_split_m(int argc, VALUE *argv, VALUE str)
 	else if (lim == 1) {
 	    if (RSTRING_LEN(str) == 0)
 		return rb_ary_new2(0);
-	    return rb_ary_new3(1, str);
+	    return rb_ary_new3(1, rb_str_dup(str));
 	}
 	i = 1;
     }

test/ruby/test_string.rb

@@ -1686,6 +1686,11 @@ CODE
     }
   end
 
+  def test_split_dupped
+    s = "abc"
+    s.split("b", 1).map(&:upcase!)
+    assert_equal("abc", s)
+  end
   def test_squeeze
     assert_equal(S("abc"), S("aaabbbbccc").squeeze)
     assert_equal(S("aa bb cc"), S("aa   bb      cc").squeeze(S(" ")))