Replace is_array/is_string with a more performant approach

Instead of doing is_array( $msg ) use (array)$msg === $msg

See also [1]

## Comparison based on available unit tests

(array)$msg === $msg + (string)$msg === $msg  ->

1) SMW\Test\MessageFormatterTest::testNewFromArray with data set #1 (array('Foo', 'Bar', array('FooBar', array('barFoo', 'Foo'))), 4)
This test printed output:
double(3.0994415283203E-5)
double(0.00034093856811523)
double(0.00063109397888184)
double(2.288818359375E-5)
double(0.00026392936706543)
double(0.00048208236694336)
double(2.288818359375E-5)
double(0.00026202201843262)
double(0.00049209594726562)

23) SMW\Test\MessageFormatterTest::testEscapedUnescaped
This test printed output:
double(2.6941299438477E-5)
double(2.5033950805664E-5)

is_array( $msg ) + is_string( $msg ) ->

1) SMW\Test\MessageFormatterTest::testNewFromArray with data set #1 (array('Foo', 'Bar', array('FooBar', array('barFoo', 'Foo'))), 4)
This test printed output:
double(6.0081481933594E-5)
double(0.0003960132598877)
double(0.00072503089904785)
double(4.9114227294922E-5)
double(0.00031900405883789)
double(0.00063800811767578)
double(4.8160552978516E-5)
double(0.0003049373626709)
double(0.00055289268493652)

23) SMW\Test\MessageFormatterTest::testEscapedUnescaped
This test printed output:
double(4.2915344238281E-5)
double(3.8862228393555E-5)

[1] https://drupal.org/node/1141770

Change-Id: I7ee8d7024f0ecf838d1a5b3f2d6ae0c063b20a50