Make sure the document processed in background.js has a title element.

[WetHat]

This addresses issue #310: which describes a download failure for a specific website, because pageTitle is empty.

The proposed fix here makes sure that the HTML document processed in background.js has a proper <title> tag, so that pageTitle can be populated.

Note: Despite a missing <title> tag browsers manage to provide a useful window title. Hence the best place to fix this issue seems to be contentScript.js because it is run in the context of the original HTML page

[aancw]

I think this PR can be adapted in the obsidian feature too.