Remove ineffective multipart option

The code looks like it is doing a multipart request, but it isn't. It is just setting the header 'Multipart' to true. I noticed this while capturing requests between an application and Fedora.

Keeping the code as is because changing it to use multipart encoding with a PUT method triggers a Fedora bug. (see https://jira.duraspace.org/browse/FCREPO-1206).