Allow Swift to set content type

This PR fixes the case where jclouds is assuming the blob content type to be `application/unknown`. The original discussion can be found [here](http://markmail.org/thread/qdxqrkhnjygjny4k).

If the user does not specify a content type, then the `X-Detect-Conent-Type` header should be added to the request headers to indicate that the server should detect the content type. 

This feature is supported in the Swift 1.9.1 through 1.13.1rc1 [releases](https://bugs.launchpad.net/openstack-api-site/+bug/1274988).

A test case is in the works now and I wanted to get some other eyes on it. DO NOT MERGE.