Emit 501 for initiateMultipartUpload in azureblob-sdk

The SDK lacks a method to upload a part with a non-repeatable payload.
References #708.

src/main/java/org/gaul/s3proxy/azureblob/AzureBlobStore.java

@@ -499,9 +499,12 @@ public final class AzureBlobStore extends BaseBlobStore {
     @Override
     public MultipartUpload initiateMultipartUpload(String container,
             BlobMetadata blobMetadata, PutOptions options) {
+/*
         String uploadId = UUID.randomUUID().toString();
         return MultipartUpload.create(container, blobMetadata.getName(),
                 uploadId, blobMetadata, options);
+*/
+        throw new UnsupportedOperationException();
     }
 
     @Override