title: Shopify API returns 400 bad request when creating new custom collection
body: <p>I'm trying to create a custom collection following the <a href="https://shopify.dev/api/admin-rest/2021-07/resources/customcollection#%5Bpost%5D/admin/api/2021-07/custom_collections.json" rel="nofollow noreferrer">document</a>. I'm getting a <code>400</code> about the request being malformed.</p>
  <p>My code:</p>
  <pre><code>require('dotenv').config()
  const { Shopify } = require('@shopify/shopify-api')
  
  const client = new Shopify.Clients.Rest(
      process.env.SHOPIFY_STORE, 
      process.env.SHOPIFY_API_SECRET_KEY
  );
  
  (async () =&gt; {
      try {
          const data = await client.post({
              path: 'custom_collections',
              body: { 'custom_collection': { 'title': 'Nike' } },
              type: 'application/json',
          })
      } catch(err) {
          console.error(JSON.stringify(err))
      }
  })()
  </code></pre>
  <p>And the error response from Shopify is:</p>
  <pre><code>{&quot;code&quot;:400,&quot;statusText&quot;:&quot;Bad Request&quot;}
  </code></pre>
  <p>Any ideas what I'm doing wrong here? My tokens are correct because I can use the same token to view collections using the https URL <code>https://API_KEY:SECRET@SHOPIFY_STORE/admin/api/2021-07/collections.json</code></p>