Fix for invalid initial state of certain sources

Certain tile sources derived from `ol.source.TileImage` (`ol.source.BingMaps` and `ol.source.TileJSON`) are passing `state: ol.source.State.LOADING` as an constructor option to the `ol.source.TileImage`, but this options is not recognized (it is not handled at all). As a result of this, the state of these tile sources is initially set to "ready", which is not desired.
`ol.source.State` is not exported; outside of the library you will use a string for the state.