http_response: allow nil values in response headers
This blatantly violates Rack SPEC, but we've had this bug since
March 2009[1]. Thus, we cannot expect all existing applications
and middlewares to fix this bug and will probably have to
support it forever.
Unfortunately, supporting this bug contributes to application
server lock-in, but at least we'll document it as such.
[1] commit
1835c9e2e12e6674b52dd80e4598cad9c4ea1e84
("HttpResponse: speed up non-multivalue headers")
Reported-by: Owen Ou <o@heroku.com>
Ref: <CAO47=rJa=zRcLn_Xm4v2cHPr6c0UswaFC_omYFEH+baSxHOWKQ@mail.gmail.com>