Added test for ampersands escaping

Will leave both double and single quotes unconverted.

Catch by @ErikMinekus

Code:

```php
$code = new Decoda\Decoda('[url]http://milesj.me?a=b&c[/url]');
$code->defaults();
echo $code->parse();
```

Output:

```html
<a href="http://milesj.me?a=b&amp;c">http://milesj.me?a=b&c</a>
```

As you can see the ampersand is correctly encoded within attributes, but not inside the content.
Even though browsers render this correctly, it's not valid HTML.
Fixed ampersands escaping in block contents

Escape special chars without quotes in block contents.

With this patch, PHP will not encode existing html entities.

Maybe will be better to add a `charset` configuration to select the good one.