Ignore If_modified_since even if if_none_match field's value is empty.

[scw00]

https://tools.ietf.org/html/rfc7232#section-3.3

A recipient MUST ignore If-Modified-Since if the request contains an If-None-Match header field;

IIUC we should ignore If-Modified-Since once the request contains If-None-Match header field. Although the value is empty.

[bryancall]

Look like this was fixed in #5701 and merged