Add contentType param to Bucket.create(blob, content, options)

[mziccard]

This was discussed with @aozarov in #251.
Since contentType is a required field we should add it to Bucket's create(blob, content, options):

Blob create(String blob, byte[] content, String contentType, BlobTrargetOptions... options)

If contentType is set to null the default content type is used, and we can document this in the javadoc.
I propose we use application/octet-stream as default.