Fix tile continually expiring when expireDate is set

[lilleyse]

For some reason I had not tested setting just expireDate without expireDuration.

This fix prevents the tile from thinking it is expired every frame.

[pjcozzi]

CI has a test failure:

1) tile expires
     Scene/Cesium3DTileset
     Expected false to be true.
    at Specs/Scene/Cesium3DTilesetSpec.js:2600:50
    at Promise.then (Source/ThirdParty/when.js:196:34)
    at Source/ThirdParty/when.js:297:13
    at processQueue (Source/ThirdParty/when.js:647:4)
    at _resolve (Source/ThirdParty/when.js:333:4)

https://travis-ci.org/AnalyticalGraphicsInc/cesium/builds/239785849?utm_source=github_status&utm_medium=notification

[lilleyse]

That fix wasn't adding the timestamp to the url - this should do it now.