In get_blob_properties of blob_service, use :head instead of :get

According to:

http://msdn.microsoft.com/en-us/library/windowsazure/dd179394.aspx

one should use HEAD method to get blob properties. However, in the get_blob_properties method of

lib/azure/blob/blob_service.rb

:get was used instead of :head. The drawback is that when an entire blob is downloaded for getting the blob properties. Calling :head avoids it.