Support for HTTP Method Overrides

HTTP Method Overrides is not supported. I was testing Azurite with Microsoft Azure Storage SDK for .NET and found out that Insert or Merge entity operations were handled as a Insert entity operation because of POST method type. When this happen incorrect result code is returned - 201 Created instead of 204 No content.

```
POST http://127.0.0.1:10002/devstoreaccount1/WebhookLogs(PartitionKey='WebhookLogs',RowKey='2518587242166512894') HTTP/1.1
User-Agent: Azure-Storage/8.0.1 (.NET CLR 4.0.30319.42000; Win32NT 10.0.17134.0)
x-ms-version: 2016-05-31
Accept-Charset: UTF-8
MaxDataServiceVersion: 3.0;NetFx
Accept: application/json;odata=minimalmetadata
X-HTTP-Method: MERGE
Content-Type: application/json
DataServiceVersion: 3.0;
x-ms-client-request-id: 4cf14138-abc1-4900-91ca-cd1aa37064b6
x-ms-date: Fri, 30 Nov 2018 11:16:30 GMT
Authorization: SharedKey devstoreaccount1:XlKwrrAuFf+Zunycy2gl91KgwykmGIXdmtBYZgzMYZ4=
Host: 127.0.0.1:10002
Content-Length: 1105
```