message: Fix superfluous '+' in view source
diff: @@ -202,7 +202,7 @@ export default function embed(el: HTMLBaseElement | string, spec: any, opt: Embe
  }
  function viewSource(source: string, sourceHeader: string, sourceFooter: string) {
  - const header = `<html><head>${sourceHeader}</head>' + '<body><pre><code class="json">`;
  + const header = `<html><head>${sourceHeader}</head><body><pre><code class="json">`;
  const footer = `</code></pre>${sourceFooter}</body></html>`;
  const win = window.open('');
  win.document.write(header + source + footer);
  