Loaded/preloaded/autoplayed videos are uncached and time out if `autoplay="false"` but missing `preload="auto"`

Description:

If a video is already loaded…

1. the video loads but is not cached.

2. an uncaught Promise is thrown (see error in the Console).
   https://github.com/aframevr/aframe/blob/68e6c26/src/core/a-assets.js#L162

3. have mediaElementLoaded return a Promise.resolve() instead of undefined.

   Change this:

     if (!el.hasAttribute('autoplay') && el.getAttribute('preload') !== 'auto') {
       return;
     }
   

   to this:

     if (!el.autoplay && el.preload !== 'auto') {
       THREE.Cache.files[el.src] = el;
       return Promise.resolve();
     }
   

   Notice the hasAttribute and getAttribute calls have been changed to instead use the <video> element's (magical) getter properties. (I commented on this in https://github.com/aframevr/aframe/issues/3173#issuecomment-337191996.)

4. Promise.all should be caught if anything throws.

/cc @machenmusik

• A-Frame Version: 0.7.0/master
• Platform / Device: any (but tested with iPhone 5 on iOS 10.3.1, iPhone 7 with iOS 11.

Start in src/core/a-assets.js, reading mediaElementLoaded around the linked lines and the Promise.all call near line 59. Verify that already-loaded video elements are cached, return a resolved promise when appropriate, and that Promise.all errors are caught without an uncaught console rejection.