Both the audio and video sanitizers use the same routine to iterate
through the source elements within the main node and filter/sanitize
their attributes. However, the filtered attributes are not added back to
the source element before it is appended to the new amp-audio or
amp-video element. This is problematic when the attribute filtering
method converts URLs from http to https, but the converted URL does not
make it back into the source element.

This commit addresses the issue by updating each source element's
attributes before it is appended.

If an audio or video element, or its source child element, has a src
URL, but it is invalid (eg it uses the http protocol instead of https),
this creates a fallback blockquote element to replace the original that
explains that the media could not be loaded.

Creates a fallback blockquote element to replace an iframe that doesn't
have a valid src attribute.

For a URL with a relative protocol, such as

//www.youtube.com/watch?v=dQw4w9WgXcQ

...the assumption is that either http or https will work with it. Since
AMP requires https, we should go ahead and add the https scheme to these
during filtering, if the sanitizer is configured to require or force
the protocol.

In the previous commit I removed the embed-specific class name from the
abstracted `create_fallback_node` method, but forgot to update the tests