Cannot find tags that have additional classes

`find` method cannot find tag that has additional classes.

For example, I want to find all tags that have 'services' class:

`<div class='services'>` or
`<div class='services last-item'>` or
`<div class='services active'>`   

But, If I run:
```php
$html->find('div[class=services]'); 
``` 
I will only get one result:
    
    <div class='services'>

