Enable ProjectionLike on getGetFeatureInfoUrl

I think as a general rule all projection api params/options should be ProjectionLike and not require that apps create a Projection object. TileWMS.getGetFeatureInfoUrl (what an ugly name!) is one that isn't, so this PR changes that.

This means that examples/getfeatureinfo can be simplified (remove a type cast).

If there are other params/options that require a Projection object, I think they should also be changed.
This changes the type of the projection variable. Can you use a new variable for the return value of ol.proj.get? Once done we can merge.