Improve perf of `create_dimensions_shorthand!`

While profiling our digest emailer I noticed a bottleneck in css_parser. Here's a simple fix.

```ruby
require 'benchmark'

@declarations = ["hello-world", "zefro"] * 1000

Benchmark.bm do |bm|
  bm.report do
    10.times do
      {'margin'       => 'margin-%s',
       'padding'      => 'padding-%s',
       'border-color' => 'border-%s-color',
       'border-style' => 'border-%s-style',
       'border-width' => 'border-%s-width'}.each do |property, expanded|

        foldable = @declarations.select do |dim, val|
          dim == expanded % 'top' or dim == expanded % 'right' or dim == expanded % 'bottom' or dim == expanded % 'left'
        end
      end
    end
  end

  bm.report do
    10.times do
      {'margin'       => 'margin-%s',
       'padding'      => 'padding-%s',
       'border-color' => 'border-%s-color',
       'border-style' => 'border-%s-style',
       'border-width' => 'border-%s-width'}.each do |property, expanded|

        right = expanded % 'right'
        top = expanded % 'top'
        left = expanded % 'left'
        bottom = expanded % 'bottom'
        foldable = @declarations.select do |dim, val|
          dim == top or dim == right or dim == bottom or dim == left
        end
      end
    end
  end

  bm.report do
    10.times do
      {'margin'       => 'margin-%s',
       'padding'      => 'padding-%s',
       'border-color' => 'border-%s-color',
       'border-style' => 'border-%s-style',
       'border-width' => 'border-%s-width'}.each do |property, expanded|

        properties = ['top', 'right', 'bottom', 'left'].map { |dir| expanded % dir }
        foldable = @declarations.select do |dim, val|
          properties.include? dim
        end
      end
    end
  end
end
# >>        user     system      total        real
# >>    0.360000   0.000000   0.360000 (  0.362463)
# >>    0.030000   0.000000   0.030000 (  0.022585)
# >>    0.030000   0.000000   0.030000 (  0.036982)
```