[Runtime] Add author href attribute check in widget element handler.

When parsing a widget config.xml file, runtime should make sure the author href
attribute is a valid URI, as it's required in W3C Widget spec. This patch will
fix this.

BUG=XWALK-2995
That's the same question I asked in #2631 -- at least the unit test depends on it, but it's still not clear whether it's the unit test or the code that needs to be changed.