Optimise CSS feature does not allow whitespace

The optimise CSS performance feature does not allow whitespace in URLs.

The CSS specification allows whitespace to appear between the parenthesis and URL value, e.g. the following are all equivalent:

url("images/pic.png")
url('images/pic.png')
url(images/pic.png)
url( "images/pic.png" )
url( 'images/pic.png' )
url( images/pic.png )

Currently, the drupal_build_css_cache method fails to match URLs containing whitespace which breaks stylesheets with URLs with whitespace. The expected behaviour is for the whitespace to be handled correctly.

The attached patch corrects this behaviour.