recur_expansion.js: fix iterator with RDATE-only recurrence

kewisch#534

When iterating with
  e is an event-component
  if (e.isRecurring() {
     let i = e.iterator();
     while (n = i.next()) {
       o = g.getOccurrenceDetails(obj)
       …
     }
  }

on the first iteration the DTSTART instance shall be returned.  It is returned,
when there is RRULE.  In the lack of this change, on the first iteration, when
RDATE is present without RRULE, the first RDATE instance is returned.

lib/ical/recur_expansion.js

@@ -106,7 +106,7 @@ class RecurExpansion {
    * @type {Number}
    * @private
    */
-  ruleDateInc = 0;
+  ruleDateInc = -1;
 
   /**
    * Current position in exDates array
@@ -240,6 +240,14 @@ class RecurExpansion {
       // _after_ we choose a value this should be
       // the only spot where we need to worry about the
       // end of events.
+      if (this.ruleDateInc == -1) {
+        this._nextRuleDay();
+        if (!iter) {
+          // on the first iteration return DTSTART
+          return this.last;
+        }
+      }
+
       if (!next && !iter) {
         // there are no more iterators or rdates
         this.complete = true;
@@ -278,7 +286,7 @@ class RecurExpansion {
       }
 
       //XXX: The spec states that after we resolve the final
-      //     list of dates we execute exdate this seems somewhat counter
+      //     list of dates we execute exdate. This seems somewhat counter
       //     intuitive to what I have seen most servers do so for now
       //     I exclude based on the original date not the one that may
       //     have been modified by the exception.
@@ -379,15 +387,14 @@ class RecurExpansion {
 
         this.ruleDateInc = 0;
         this.last = this.ruleDates[0].clone();
+        this.ruleDate = this.ruleDates[0];
       } else {
         this.ruleDateInc = binsearchInsert(
           this.ruleDates,
           this.last,
           (a, b) => a.compare(b)
-        );
+        ) - 1;
       }
-
-      this.ruleDate = this.ruleDates[this.ruleDateInc];
     }
 
     if (component.hasProperty('rrule')) {

test/recur_expansion_test.js

@@ -54,6 +54,29 @@ suite('recur_expansion', function() {
       }, ".ruleIterators or .component must be given");
     });
 
+    test('only rdate without rrule', function() {
+      let component = primary.component.toJSON();
+      component = new ICAL.Component(component);
+      component.removeAllProperties('rrule');
+
+      const subj = new ICAL.RecurExpansion({
+        component,
+        dtstart: primary.startDate
+      });
+      let expected = [
+        new Date(2012, 9, 2, 10),
+        new Date(2012, 10, 5, 10),
+        new Date(2012, 10, 10, 10),
+        new Date(2012, 10, 30, 10)
+      ], dates = [], next;
+
+      while ((next = subj.next())) {
+        dates.push(next.toJSDate());
+      }
+
+      assert.deepEqual(dates, expected);
+    });
+
     test('default', function() {
       let dtstart = ICAL.Time.fromData({
         year: 2012,