title: "Incorrectly formatted 'headers' entry" when setting Last-Modified header
body: <p>I'm trying to set the <code>Last-Modified</code> header for my resources so they can be properly cached. However, when I run <code>firebase deploy</code> I get the following error:</p>
  
  <pre class="lang-none prettyprint-override"><code>FIREBASE WARNING: set at /hosting/headers/dstaley failed: permission_denied
  Settings Error - Incorrectly formatted "headers" entry in the firebase.json
  </code></pre>
  
  <p>My firebase.json is as follows:</p>
  
  <pre class="lang-json prettyprint-override"><code>{
    "firebase": "dstaley",
    "public": "dist",
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
    "rewrites": [
      {
        "source": "**",
        "destination": "/index.html"
      }
    ],
    "headers": [
      {
        "source": "**/*.@(js|css)",
        "headers": [
          {
            "key": "Cache-Control",
            "value": "max-age=31536000"
          }
        ]
      },
      {
        "source": "**/*.@(js|css|html)",
        "headers": [
          {
            "key": "Last-Modified",
            "value": "Thu, 28 Aug 2014 16:25:05 GMT"
          }
        ]
      }
    ]
  }
  </code></pre>
  
  <p>The file looks properly formatted, and the <code>permission_denied</code> error leads me to believe that it's not possible to set the <code>Last-Modified</code> header with Firebase hosting. Any ideas?</p>