fix: use enums instead of str for task creation (GoogleCloudPlatform#4664)

Closes GoogleCloudPlatform#4653 GoogleCloudPlatform#4652 GoogleCloudPlatform#4651 GoogleCloudPlatform#4650 GoogleCloudPlatform#4649

Author: busunkim96

appengine/flexible/tasks/create_app_engine_queue_task.py

@@ -39,7 +39,7 @@ def create_task(project, queue, location, payload=None, in_seconds=None):
     # Construct the request body.
     task = {
             'app_engine_http_request': {  # Specify the type of request.
-                'http_method': 'POST',
+                'http_method': tasks_v2.HttpMethod.POST,
                 'relative_uri': '/example_task_handler'
             }
     }

appengine/flexible/tasks/snippets.py

@@ -90,7 +90,7 @@ def create_task(project, location, queue):
 
     task = {
         'app_engine_http_request': {
-            'http_method': 'POST',
+            'http_method': tasks.HttpMethod.POST,
             'relative_uri': '/update_counter',
             'app_engine_routing': {
                 'service': 'worker'
@@ -120,7 +120,7 @@ def create_tasks_with_data(project, location, queue):
 
     task1 = {
         'app_engine_http_request': {
-            'http_method': 'POST',
+            'http_method': tasks.HttpMethod.POST,
             'relative_uri': '/update_counter?key=blue',
             'app_engine_routing': {
                 'service': 'worker'
@@ -130,7 +130,7 @@ def create_tasks_with_data(project, location, queue):
 
     task2 = {
         'app_engine_http_request': {
-            'http_method': 'POST',
+            'http_method': tasks.HttpMethod.POST,
             'relative_uri': '/update_counter',
             'app_engine_routing': {
                 'service': 'worker'
@@ -165,7 +165,7 @@ def create_task_with_name(project, location, queue, task_name):
     task = {
         'name': client.task_path(project, location, queue, task_name),
         'app_engine_http_request': {
-            'http_method': 'GET',
+            'http_method': tasks.HttpMethod.GET,
             'relative_uri': '/url/path'
         }
     }