Description:
------------
MessageFormatter::format() accepted named arguments to create strings, but MessageFormatter::parse() dont output named array indices.

Test script:
---------------
$messageFormatter = new MessageFormatter(NULL, 'foo {0}, bar {1}');
$output = $messageFormatter->format(array(
    23,
    42
));
var_dump($output);
var_dump($messageFormatter->parse($output));


$messageFormatter = new MessageFormatter(NULL, 'foo {foo}, bar {bar}');
$output = $messageFormatter->format(array(
    'foo' => 23,
    'bar' => 42
));
var_dump($output);
var_dump($messageFormatter->parse($output));


Expected result:
----------------
string(14) "foo 23, bar 42"
array(2) {
  [0]=>
  string(2) "23"
  [1]=>
  string(2) "42"
}

string(14) "foo 23, bar 42"
array(2) {
  [foo]=>
  string(2) "23"
  [bar]=>
  string(2) "42"
}


Actual result:
--------------
string(14) "foo 23, bar 42"
array(2) {
  [0]=>
  string(2) "23"
  [1]=>
  string(2) "42"
}

string(14) "foo 23, bar 42"
bool(false)