title: serverMiddleware access Nuxt instance or context
body: <p>im trying to access Nuxt runtime variables from a serverMiddleware</p>
  <p>Example i have this <strong>context.$db</strong> which i added from this plugin</p>
  <p><strong>nuxt.config.js</strong>:</p>
  <pre><code>
      plugins: [
          { src: '~/plugins/db_runtime.js', mode: 'server' }
      ]
  
  </code></pre>
  <p><strong>~/plugins/db_runtime.js</strong>:</p>
  <pre><code>
      db = 'test'
      
      export default ({ app }, inject) =&gt; {
          inject('db', db)
      }
  
  </code></pre>
  <p>Then i added a serverMiddleware:</p>
  <pre><code>
      serverMiddleware: [
          { path: '/api', handler: '~/api/index.js' },
      ],
  
  </code></pre>
  <p>The serverMiddleware: <strong>~/api/index.js</strong>:</p>
  <pre><code>
      export default function(res, req) {
      }
  
  </code></pre>
  <p>Is there anyway to access <strong>context.$db</strong> from there?
  i.e</p>
  <pre><code>    export default function(res, req) {
          $config.db = null
      }
  </code></pre>