Bug 607616 — Permit the uploading of documents of any type

Add support to GDataDocumentsUploadQuery for specifying that documents
shouldn’t be converted to common formats on being uploaded. This permits
arbitrary files to be uploaded.

This includes test cases.

API additions:
 • GDataDocumentsUploadQuery:convert,
   gdata_documents_upload_query_get_convert and
   gdata_documents_upload_query_set_convert

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=607616