Replace header when sending first one of each header (for 4.x)

[akrabat]

Forward port #2441 to 4.x:

When we call header() to send the headers in the Response, set the replace parameter to true for the first of that header name and then set it to false.

Don't do this if the header is Set-Cookie so we don't break sessions.

Fixes #2282
Fixes #2246

[coveralls]

Coverage increased (+1.3%) to 97.799% when pulling ed6a18f on akrabat:override-previously-set-headers-4.x into 6ec52e3 on slimphp:4.x.