Web Site Extension installation returns Bad Request when called without content

As suggested by @hovsepm [here](https://github.com/Azure/azure-libraries-for-net/issues/241#issuecomment-378684495), Azure SDK calls fail to create site extensions when they attempt to PUT a request with null content. Setting request body to an empty JSON object (`{}`) fixes the problem.

No file or test is named. Locate the Web Site Extension installation PUT operation in the Azure REST API specifications, inspect how its null request body is defined, and verify that the specification requires an empty JSON object so the request no longer returns Bad Request.

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