Check if contentType and contentTypeHeader is already set

Hi,

We have used your grunt task for uploading to Azure blob storage, and it worked great. 
However when we tried to set content-type via metadata (to add a charset), it didnt work.

This is a fix for that, to just check if the meta is already set and avoid overwriting it with the automatic lookup.
Maybe there is a better solution to add charset to the Content-Type header that you know of, but otherwise i hope you can accept this fix so that we can continue to use your version.