Fix the timeZone param parsing.

src/main/kotlin/org/motivepick/web/ScheduleController.kt

@@ -5,12 +5,14 @@ import org.motivepick.service.TaskService
 import org.springframework.http.ResponseEntity
 import org.springframework.http.ResponseEntity.ok
 import org.springframework.web.bind.annotation.GetMapping
+import org.springframework.web.bind.annotation.RequestParam
 import org.springframework.web.bind.annotation.RestController
 import java.time.ZoneId
 
 @RestController
 internal class ScheduleController(private val taskService: TaskService) {
 
     @GetMapping("/schedule")
-    fun schedule(timeZone: ZoneId?): ResponseEntity<ScheduleView> = ok(taskService.findScheduleForCurrentUser(timeZone ?: ZoneId.of("UTC")))
+    fun schedule(@RequestParam(name = "timeZone", defaultValue = "UTC") timeZone: String): ResponseEntity<ScheduleView> =
+        ok(taskService.findScheduleForCurrentUser(ZoneId.of(timeZone)))
 }