Implemented the donation response

ref #66

src/models/entities/DistanceDonation.ts

@@ -43,7 +43,7 @@ export class DistanceDonation extends Donation {
   /**
    * Turns this entity into it's response class.
    */
-  public toResponse() {
-    return new Error("NotImplemented");
+  public toResponse(): DistanceDonation {
+    return null;
   }
 }