Allow ol.proj.Projection units option to be a string

src/objectliterals.jsdoc

@@ -82,7 +82,7 @@
  * Object literal with config options for the projection.
  * @typedef {Object} olx.ProjectionOptions
  * @property {string} code The SRS identifier code, e.g. `EPSG:4326`.
- * @property {ol.proj.Units} units Units.
+ * @property {ol.proj.Units|string} units Units.
  * @property {ol.Extent|undefined} extent The validity extent for the SRS.
  * @property {string|undefined} axisOrientation The axis orientation as
  *     specified in Proj4. The default is `enu`.

src/ol/proj/proj.js

@@ -77,7 +77,7 @@ ol.proj.Projection = function(options) {
    * @private
    * @type {ol.proj.Units}
    */
-  this.units_ = options.units;
+  this.units_ = /** @type {ol.proj.Units} */ (options.units);
 
   /**
    * @private