resource/pagerduty_maintenance_window: Suppress spurious diff

[radeksimko]

This is to fix the following test failures:

=== RUN   TestAccPagerDutyMaintenanceWindow_Basic
--- FAIL: TestAccPagerDutyMaintenanceWindow_Basic (5.34s)
    testing.go:459: Step 0 error: After applying this step and refreshing, the plan was not empty:
        
        DIFF:
        
        UPDATE: pagerduty_maintenance_window.foo
          end_time:   "2019-03-12T04:36:03+00:00" => "2019-03-12T04:36:03Z"
          start_time: "2019-03-11T04:36:03+00:00" => "2019-03-11T04:36:03Z"
        
=== RUN   TestAccPagerDutyMaintenanceWindow_import
--- FAIL: TestAccPagerDutyMaintenanceWindow_import (5.43s)
    testing.go:459: Step 0 error: After applying this step and refreshing, the plan was not empty:
        
        DIFF:
        
        UPDATE: pagerduty_maintenance_window.foo
          end_time:   "2019-03-12T04:34:57+00:00" => "2019-03-12T04:34:57Z"
          start_time: "2019-03-11T04:34:57+00:00" => "2019-03-11T04:34:57Z"