fix(storage): accept the official format for $STORAGE_EMULATOR_HOST (…

…#632)

In order for this library to be compatible with the official python Google Cloud
Storage library, it needs to accept the same value for configuring the
`STORAGE_EMULATOR_HOST` environment variable. This will allow users to
rely on both libraries in a single project.

---------

Co-authored-by: Kevin James <kevin@talkiq.com>

storage/gcloud/aio/storage/__init__.py

@@ -107,7 +107,7 @@
 
 ```shell
 docker run -d -p 4443:4443 -v $PWD/my-sample-data:/data fsouza/fake-gcs-server
-export STORAGE_EMULATOR_HOST='0.0.0.0:4443'
+export STORAGE_EMULATOR_HOST='http://0.0.0.0:4443'
 ```
 
 Any `gcloud-aio-storage` requests made with that environment variable set will

storage/gcloud/aio/storage/storage.py

@@ -5,6 +5,7 @@
 import logging
 import mimetypes
 import os
+import warnings
 from typing import Any
 from typing import AnyStr
 from typing import Dict
@@ -51,7 +52,11 @@ def init_api_root(api_root: Optional[str]) -> Tuple[bool, str]:
 
     host = os.environ.get('STORAGE_EMULATOR_HOST')
     if host:
-        return True, f'http://{host}'
+        if not host.startswith('http'):
+            warnings.warn('STORAGE_EMULATOR_HOST must include http:// prefix',
+                          DeprecationWarning)
+            host = f'http://{host}'
+        return True, host
 
     return False, 'https://www.googleapis.com'