Just noticed the whitespace scenario is mentioned at <https://tools.ietf.org/html/rfc7230#section-3.2.4>:

'''
No whitespace is allowed between the header field-name and colon.  In the past, differences in the handling of such whitespace have led to security vulnerabilities in . . . response handling.  . . .  A proxy must remove any such whitespace from a response message before forwarding the message downstream.
'''

It would not be possible build a proxy that does that using Python 3’s current HTTP client.