Fix bug with '=' present in shortcode attribute value
Fix bug with '=' present in shortcode attribute value

Eg [sc url="https://www.youtube.com/watch?v=Eo2Zp-Lce1I"]
Would be parsed into (attrs):
array('url=https://www.youtube.com/watch?v' => 'Eo2Zp-Lce1I')
sheadawson/silverstripe-shortcodable,sheadawson/silverstripe-shortcodable