Fixed parsing problem when using syck extension.

When using syck extension and load from a string, syck_load() throws SyckException because the argument is single-line.

repro:
```php
require_once 'Spyc.php';

$yaml = <<<END_YAML
example:
  foo: bar
  bar: baz
END_YAML;

$spyc = new Spyc();
$spyc->setting_use_syck_is_possible = true;
$spyc->load($yaml);		// SyckException
```