Fix dateTimeThisYear method (#2039)

Author: macaron

Diff for: src/Faker/Provider/DateTime.php

@@ -202,7 +202,7 @@ public static function dateTimeThisDecade($max = 'now', $timezone = null)
      */
     public static function dateTimeThisYear($max = 'now', $timezone = null)
     {
-        return static::dateTimeBetween('-1 year', $max, $timezone);
+        return static::dateTimeBetween('first day of january this year', $max, $timezone);
     }
 
     /**

Diff for: test/Faker/Provider/DateTimeTest.php

@@ -122,7 +122,7 @@ public function testDateTimeThisYear()
     {
         $date = DateTimeProvider::dateTimeThisYear();
         $this->assertInstanceOf('\DateTime', $date);
-        $this->assertGreaterThanOrEqual(new \DateTime('-1 year'), $date);
+        $this->assertGreaterThanOrEqual(new \DateTime('first day of january this year'), $date);
         $this->assertLessThanOrEqual(new \DateTime(), $date);
         $this->assertEquals(new \DateTimeZone($this->defaultTz), $date->getTimezone());
     }