Update AzureBlobStore.java

JCLOUDS-391: 
set user-defined metadata for Azure with multipart blobs
Looking at [this comment](https://issues.apache.org/jira/browse/JCLOUDS-391?focusedCommentId=14032527&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14032527), this looks like "the patch way" rather than the "proper" solution. That's fine, but should we add a comment to indicate that the "more proper" way to solve this would be to do X..?

/cc @adriancole