Allow 304 responses with a Content-Length but no body

[Tratcher]

304 Not Modified responses are not allowed to contain a body, but they may still contain an informative Content-Length header similar to a Head response.

https://tools.ietf.org/html/rfc7230#section-3.3.2

A server MAY send a Content-Length header field in a 304 (Not
Modified) response to a conditional GET request (Section 4.1 of
[RFC7232]); a server MUST NOT send Content-Length in such a response
unless its field-value equals the decimal number of octets that would
have been sent in the payload body of a 200 (OK) response to the same
request.
A server MUST NOT send a Content-Length header field in any response
with a status code of 1xx (Informational) or 204 (No Content). A
server MUST NOT send a Content-Length header field in any 2xx
(Successful) response to a CONNECT request (Section 4.3.6 of
[RFC7231]).

MVC started doing this in 2.0 and running into the Response Content-Length mismatch: too few bytes written (0 of 42). error.
aspnet/Mvc#6875
They've patched it for 2.0.1
aspnet/Mvc#6887