fix: handle CSS functions in a CSS shorthand property

[flavorjones]

See #199 for context. The following works as expected:

Loofah::fragment(%Q[<h1 style="background-color: linear-gradient(transparent 50%, #ffff66 50%); ">here</h1>]).scrub!(:prune)
# => <h1 style="background-color: linear-gradient(transparent 50%, #ffff66 50%);">here</h1>

Loofah::fragment(%Q[<h1 style="color: linear-gradient(transparent 50%, #ffff66 50%); ">here</h1>]).scrub!(:prune)
# => <h1 style="color: linear-gradient(transparent 50%, #ffff66 50%);">here</h1>

but this does not:

Loofah::fragment(%Q[<h1 style="background: linear-gradient(transparent 50%, #ffff66 50%); ">here</h1>]).scrub!(:prune)
# => <h1 style="background:#ffff66;">here</h1>

This fix simplifies how we handle CSS properties and their values. I'm still not totally comfortable with the fact that we check each value of a CSS shorthand property differently than the longer form, but that's for another time.