Object metadata keys are lowercased when uploading to GCS

## Summary

Mixed-case metadata is lowercased when using libcloud to upload objects to Google Cloud Storage.

## Detailed Information

I'm using libcloud to upload files to a Google Cloud Storage bucket together with object metadata.
In the process, the keys in my metadata dict are being lowercased. This is not due to Google Cloud Storage, which does support mixed case metadata.

The issue can be reproduced following the example from the docs:
```
from libcloud.storage.types import Provider
from libcloud.storage.providers import get_driver

cls = get_driver(Provider.GOOGLE_STORAGE)
driver = cls('SA-EMAIL', './SA.json') # provide service account credentials here

FILE_PATH = '/home/user/file'

extra = {'meta_data': {'camelCase': 'foo'}}

# Upload with metadata
with open(FILE_PATH, 'rb') as iterator:
obj = driver.upload_object_via_stream(iterator=iterator,
container=container,
object_name='file',
extra=extra)
```
The file uploads succesfully, but resulting metadata will look as follows:

Where `camelCase` has been turned into `camelcase`.

I'm using apache-libcloud==3.0.0 with Python 3.8 on Linux (the python:3.8 Docker image).

エントリーポイント upload_object_via_stream から開始し、Google Cloud Storage 用の追加メタデータ dict がどのように準備されるかを追跡します。camelCase のキーを使った例を再現し、その後、アップロードされたオブジェクトのメタデータが元の大文字と小文字の使い分けを保持していることを確認します。

索引モデルが issue の本文から書いたものです。