Server failed to authenticate error when metadata values contain trailing whitespace

### Which service(blob, file, queue) does this issue concern?
blob

### Which version of the SDK was used? Please provide the output of `pip freeze`.
azure-common==1.1.23
azure-storage-blob==2.1.0
azure-storage-common==2.1.0

### What problem was encountered?
Call to ` create_blob_from_bytes()` throws an error [azure.common.AzureHttpError] when any of the metadata values have leading or trailing whitespace.

```
azure.common.AzureHttpError: Server failed to authenticate the request. Make sure the value of Authorization header is formed correctly including the signature. ErrorCode: AuthenticationFailed
AuthenticationFailedServer failed to authenticate the request. Make sure the value of Authorization header is formed correctly including the signature.
RequestId:78143b83-701e-0007-57a6-53ca4f000000
Time:2019-08-15T20:14:22.7942254ZThe MAC signature found in the HTTP request '(redacted base64 string)' is not the same as any computed signature. Server used following string to sign: 'PUT 61932 ...'
```

### Have you found a mitigation/solution?
Yes, trim all values in the metadata dict.

Old metadata (throws error):
```
metadata = {'LDate': '6/19/2018 12:15:16 PM', 'DocName': 'Utah Contracts 1 ', 'FileExtension': 'docx'}
service.create_blob_from_bytes(container, name, content, content_settings=content_settings, metadata=metadata)
```

New metadata (no error):
```
metadata = {'LDate': '6/19/2018 12:15:16 PM', 'DocName': 'Utah Contracts 1', 'FileExtension': 'docx'}
service.create_blob_from_bytes(container, name, content, content_settings=content_settings, metadata=metadata)
```

Reproduce the failure through create_blob_from_bytes() using metadata with leading or trailing whitespace, as shown in the issue. Trace how the Azure Storage SDK authenticates and signs this request, then verify that uploads with such metadata succeed while the existing example without trailing whitespace continues to work.

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