Preserve data type of numeric strings

This patch will help prevent string to integer conversion.

Here is an example script:

```
<?php
require 'Spyc.php';
$x = "x: '0'\n";
$data = Spyc::YAMLLoad($x);
var_dump($data['x']);
$data = Spyc::YAMLLoad(Spyc::YAMLDump($data));
var_dump($data['x']);
```

Result from current implementation:

```
string(1) "0"
int(0)
```

Patched implementation:

```
string(1) "0"
string(1) "0"
```