Keyframes from used animations with multiple keyframes are removed

[ThisNameWasTaken]

Describe the bug
Let's say we have the fallowing files:
styles.css

.scale-spin {
    animation: spin 300ms linear infinite forwards,scale 300ms linear infinite alternate;
}
@keyframes spin { /* ... */ }
@keyframes scale  { /* ... */ }

index.html

<div class="scale-spin"></div>

purgecss config

new Purgecss({
    content: ['index.html'],
    css: ['styles.css'],
    keyframes: true // discard unused keyframes
})

As of right now, if you run purgecss on these two, the scale keyframe will be removed and only the spin would be kept.

Expected behavior
Both the scale and spin keyframes are kept.

Desktop (please complete the following information):

• OS: Windows 10
• Version of Purgecss: 1.1.0

I will make a pull request to fix this issue.