CSS classes in Markdown component don't merge with component's root class

[ManasMadrecha]

Version

@nuxt/content: v1.14.0
nuxt: v2.15.5

Steps to reproduce

1. Create a component abc.vue.

<template>
  <div class="level1">
   <div class="level2">
    <slot></slot>
   </div>
  </div>
</template>

2. Use it in markdown

<abc class="tw-bg-pink-100">
</abc>

What is Expected?

1. <abc> component's root should have two classes, i.e. level1 and tw-bg-pink-100.

What is actually happening?

1. <abc> component only gets the class applied in markdown, i.e. tw-bg-pink-100.
2. level1 class is simply removed.

[nozomuikuta]

Duplicate of #831?