Ensure the public properties of DatePeriod are available

Although these properties are not currently documented,
they've been publically available so we should ensure they remain so

Author: duncan3dc

ext/date/tests/DatePeriod_properties1.phpt

@@ -0,0 +1,37 @@
+--TEST--
+DatePeriod: Test read only properties
+--INI--
+date.timezone=UTC
+--FILE--
+<?php
+
+$start = new DateTime;
+$interval = new DateInterval('P1D');
+$end = new DateTime;
+$period = new DatePeriod($start, $interval, $end);
+
+echo "recurrences: ";
+var_dump($period->recurrences);
+
+echo "include_start_date: ";
+var_dump($period->include_start_date);
+
+echo "start: ";
+var_dump($period->start == $start);
+
+echo "current: ";
+var_dump($period->current);
+
+echo "end: ";
+var_dump($period->end == $end);
+
+echo "interval: ";
+var_dump($period->interval == $interval);
+?>
+--EXPECT--
+recurrences: int(1)
+include_start_date: bool(true)
+start: bool(true)
+current: NULL
+end: bool(true)
+interval: bool(true)