title: 404 error when writing to a Shopify asset
body: <p>I've been trying to upload an asset using the shopify_api gem. I made sure I have the appropriate OAuth2 scope (write_themes), and I have no problem reading and even destroying them. The problem is that I get a 404 error when attempting to create or update an asset.</p>
  
  <p>Here's the request the gem is creating:</p>
  
  <pre><code>PUT: /admin/themes/3650318/assets.json [{"Content-Type"=&gt;"application/json", "User-Agent"=&gt;"ShopifyAPI/3.0.3 ActiveResource/4.0.0.beta1 Ruby/2.0.0", "X-Shopify-Access-Token"=&gt;"ommitted"}] ({"key":"templates/index.liquid","attachment":"base64 attachment omitted"})
  </code></pre>
  
  <p>For reference, here is the code I've used to make the request (wrapped in a <code>ShopifyAPI::Session</code>, of course):</p>
  
  <pre><code>ShopifyAPI::Asset.create(key: 'snippets/test.liquid', attachment: some_base64_data, theme_id: 3650318)
  </code></pre>
  
  <p>Or:</p>
  
  <pre><code>asset = ShopifyAPI::Asset.new(key: 'snippets/test.liquid', attachment: baset64_data, theme_id: 3650318)
  asset.save
  </code></pre>
  
  <p>Any ideas?</p>