OAuth1Session image data throws error with python3

[marekrei]

I'm trying to pass image an image file as data using OAuth1Session.

On line 228 of session.py there is a following check:

rauth/rauth/session.py

Line 228 in a6d887d

if oauth_param in data:

oauth_param in that context is taken from OPTIONAL_OAUTH_PARAMS and has type 'str'.
data is the image I'm uploading and has type 'bytes'.

Checking for a string in bytes produces an exception in python3.7:

File "/home/marek/anaconda3/envs/smuploader/lib/python3.7/site-packages/rauth/session.py", line 228, in _parse_optional_params
    if oauth_param in data:
TypeError: a bytes-like object is required, not 'str'

Am I missing something or is this a bug in rauth?