Oh jeez were living in 1999 
Actually the up to date RFC is rfc7230, rfc2616 is obsolete.
Which states that:
A sender must not generate multiple header fields with the same field name in a message unless either the entire field value for that header field is defined as a comma-separated list [i.e., #(values)] or the header field is a well-known exception (as noted below).
The exception is the Set-Cookie header.
The multiple whitespace instead of single seperating " chunked" from the field name, is indeed allow but holy crap, in all my years this is the first time I see a “professional” webserver return it like that. Professional defined as the webserver for a company valued at over 1b dollars.
Indeed I have patched my HTTP client, but just bringing this to the attention of Twitch, as I am sure a crapton of other clients will give issues.