Prefer Guava HttpHeaders over JAX-RS

jclouds code contains more instances of the former and Guava has
additional constants like CONTENT_MD5.
Revert "Fixes a bug with swift where the metadata is updated, but the content-type is reset to application/x-www-form-urlencoded."

This reverts commit a68c407b414ff361e8201056482134474188770d.
Issue number missing from the commit message.
JCLOUDS-565: Fixes a bug with swift where the metadata is updated, but the content-type is reset to application/x-www-form-urlencoded.

Workaround: send an empty Content-Type header, only for the updateMetadata call.

Re-commit of a68c407b4 with issue number.