Fix monthly recurrence at end of month

app/src/main/java/com/todoroo/astrid/repeats/RepeatTaskHelper.kt

@@ -125,7 +125,7 @@ class RepeatTaskHelper @Inject constructor(
         private fun handleMonthlyRepeat(
                 original: DateTime, startDateAsDV: Date, hasDueTime: Boolean, recur: Recur): Long {
             return if (original.isLastDayOfMonth) {
-                val interval = recur.interval
+                val interval = recur.interval.coerceAtLeast(1)
                 val newDateTime = original.plusMonths(interval)
                 val time = newDateTime.withDayOfMonth(newDateTime.numberOfDaysInMonth).millis
                 if (hasDueTime) {

app/src/test/java/com/todoroo/astrid/repeats/RepeatMonthlyTests.kt

@@ -56,6 +56,30 @@ class RepeatMonthlyTests : RepeatTests() {
         assertEquals(newDayTime(2017, 2, 28, 13, 30), next)
     }
 
+    @Test
+    fun repeatMonthlyNoInterval() {
+        val task = newFromDue(
+            "FREQ=MONTHLY",
+            newDayTime(2017, 11, 1, 13, 30)
+        )
+
+        val next = calculateNextDueDate(task)
+
+        assertEquals(newDayTime(2017, 12, 1, 13, 30), next)
+    }
+
+    @Test
+    fun repeatMonthlyEndOfMonthNoInterval() {
+        val task = newFromDue(
+            "FREQ=MONTHLY",
+            newDayTime(2017, 11, 30, 13, 30)
+        )
+
+        val next = calculateNextDueDate(task)
+
+        assertEquals(newDayTime(2017, 12, 31, 13, 30), next)
+    }
+
     /* https://tools.ietf.org/html/rfc5545#section-3.3.10
      * Recurrence rules may generate recurrence instances with an invalid
      * date (e.g., February 30) or nonexistent local time (e.g., 1:30 AM