Make sure shorthand css property values in ALLOWED_CSS_KEYWORDS get marked as clean.

Example: a rule like ```background-x: center``` should be included in the sanitized output since ```center```is in ```ALLOWED_CSS_KEYWORDS```.

So for:
```ruby
clean << "#{prop}: #{val};" unless val.split().any? do |keyword|
  WhiteList::ALLOWED_CSS_KEYWORDS.include?(keyword) &&
    keyword !~ /^(#[0-9a-f]+|rgb\(\d+%?,\d*%?,?\d*%?\)?|\d{0,2}\.?\d{0,2}(cm|em|ex|in|mm|pc|pt|px|%|,|\))?)$/
end
```
The block should return false. However, it doesn't.

Likewise we expect a property like ```background-repeat: repeat``` to not be included, since ```repeat``` isn't in ```ALLOWED_CSS_KEYWORDS```. The block should return true for this to happen. It doesn't.

//cc @rafaelfranca