title: Setting proxy using firefox webextension with browserSettings.proxyConfig
body: <p>I've created a Firefox web extension through which I'm trying to change the proxy settings using web extension API. The issue here is <code>proxyConfig</code> is expecting the value of <code>http</code> or any other type to be of the form <code>http://proxy_address:port</code> and is saving the proxy details with <code>http://</code> which is a problem as the browser is storing this <code>http://</code> in place of <code>proxy_address</code> or <code>ip</code>. I want to store the proxy without this <code>http://</code> prefix. Also, if I remove the <code>http://</code> in <code>http</code> settings the browser is not storing the proxy as expected. </p>
  
  <pre><code>var proxySettings = {
      proxyType: "manual",
      http: "proxy_address:proxy_port"
  };
  browser.browserSettings.proxyConfig.set({value: proxySettings});
  </code></pre>