Add support for body parameters when uploading new file version

Currently, the SDK only supports building a multipart/form-data body using the file content only. However, the Upload File Version operation supports additional body parameters for specifying a new 'name' as well as the 'content_modified_at' attribute. (https://developer.box.com/reference#upload-a-new-version-of-a-file)

(The "BoxBase64" class itself seems to be non-standard (looks like it's blending in some "quoted-printable" encoding?, etc.), and we've been unsuccessful at trying to modify this class to support the additional body parameters...)

The workaround is to upload the new file version then rename the file in a separate request.

Start by reading the BoxBase64 class and the upload-new-file-version operation described in the issue. Determine how the multipart/form-data body currently uses only file content, then verify that it can also carry the name and content_modified_at parameters without requiring a separate rename request.

Written by the indexing model from the issue text.