Add special case for Dates in clone

Fixes #78

Author: chbrown

util.ts

@@ -22,7 +22,7 @@ function isNonPrimitive(value: any): value is object {
 /**
 Recursively copy a value.
 
-@param source - should be a JavaScript primitive, Array, or (plain old) Object.
+@param source - should be a JavaScript primitive, Array, Date, or (plain old) Object.
 @returns copy of source where every Array and Object have been recursively
          reconstructed from their constituent elements
 */
@@ -42,6 +42,11 @@ export function clone<T extends any>(source: T): T {
     }
     return arrayTarget
   }
+  // Date
+  if (source.constructor == Date) {
+    const dateTarget: any = new Date(+source)
+    return dateTarget
+  }
   // Object
   const objectTarget: any = {}
   // declaring the variable (with const) inside the loop is faster