Refactor many small issue fixes

I fixed several issues I discovered.

fixed: header Upload-Offset was missing so resumability did not worked
fixed: flask application factory pattern did not worked using initialising of flask-tus with init_app - fixed
fixed: if client send request for upload endpoint without trailing slash upload did failed
fixed: passing error message to make_base_response
in case that upload failed because it expired added message to the error
fixed: mongoengine parameters for model.create were incorrect it does not accept positional parameters: http://docs.mongoengine.org/apireference.html#mongoengine.queryset.QuerySet.create

volesen / flask-tus