Fix `String#partition`

Split with the matched part when the separator matches the empty
part at the beginning.  [Bug #11014]

string.c

@@ -9795,7 +9795,6 @@ rb_str_partition(VALUE str, VALUE sep)
 	    return rb_ary_new3(3, rb_str_dup(str), str_new_empty(str), str_new_empty(str));
 	}
 	sep = rb_str_subpat(str, sep, INT2FIX(0));
-	if (pos == 0 && RSTRING_LEN(sep) == 0) goto failed;
     }
     else {
 	pos = rb_str_index(str, sep, 0);

test/ruby/test_string.rb

@@ -2567,6 +2567,8 @@ CODE
     hello = "hello"
     hello.partition("hi").map(&:upcase!)
     assert_equal("hello", hello, bug)
+
+    assert_equal(["", "", "foo"], "foo".partition(/^=*/))
   end
 
   def test_rpartition