Using the Uploader class directly fails because of no headers

[peixian]

When using the uploader.py file like so:

from tusclient.uploader import Uploader

my_uploader = Uploader('path/to/file.ext',
                       url='http://master.tus.io/files/abcdef123456',
                       chunk_size=200)

a 'NoneType' object has no attribute 'headers' error is raised. This is because self.headers is never set, resulting in the error when self.getoffset() is called: https://github.com/tus/tus-py-client/blob/master/tusclient/uploader.py#L92

[ifedapoolarewaju]

@peixian thank you for reporting. I'll work on this, but please also feel free to submit a PR :)

[ifedapoolarewaju]

this issue is fixed here