Fix bug when checking schema properties for schemaChange

[mrxz]

Description:
Components that have one or more properties marked as schemaChange will check if any of these properties change to trigger an updateSchema. While checking the properties in an update, it incorrectly assumes each property to also exist, which might not be the case.

Reproduction (https://glitch.com/edit/#!/beaded-basalt-church?path=index.html):

<script>
    AFRAME.registerComponent('bug', {
      init: function() {
        this.el.setAttribute('material', {
            invalidProperty: 'causes an error',
        })
      }
    })
</script>
(...)
<a-scene>
    <a-entity material bug></a-entity>
</a-scene>

Changes proposed:

• Ensure a property is part of the schema before checking its schemaChange flag

[dmarcos]

Is it possible to cover the fix with a test?

[mrxz]

Added a couple unit tests covering schemaChange/udpateSchema (there were non before) including one covering the fixed behaviour.

[dmarcos]

Thanks!

[vincentfretin]

@mrxz There is a related error in the aframe-inspector demo page. I didn't have time to investigate but it's exactly on the same line you just changed.
aframevr/aframe-inspector#701