The function ol.proj.equivalent is used for comparing two projections,
but not exposed publicly. It should be accessible using a public API to
be able to compare projections in the same way as OpenLayers does.
A test for the ol.proj.equivalent function itself already exists in
https://github.com/openlayers/ol3/blob/master/test/spec/ol/proj/proj.test.js
github issue #4913
