Border classes that are specific to one or more sides override one another

[TrevorHinesley]

Describe the bug
Similar to the bug fixed in #13, border-b-*, border-t-*, border-l-*, and border-r-* override one another, but they shouldn't. Same with border-x-* and border-y-*.

To Reproduce
Tests here and here

On any element:

<div class={Tails.classes(["border-l-4 border-b-4"])}></div>

The rendered element:

<div class="border-b-4"></div>

Expected behavior
I expect the rendered element to maintain non-competing classes, like so:

<div class="border-l-4 border-b-4"></div>