Add functions for getting and setting response headers

doc/src/manual/cowboy_req.resp_header.asciidoc

@@ -0,0 +1,61 @@
+= cowboy_req:resp_header(3)
+
+== Name
+
+cowboy_req:resp_header - Access the named HTTP header set for the response
+
+== Description
+
+[source,erlang]
+----
+resp_header(Name, Req)          -> resp_header(Name, Req, undefined)
+resp_header(Name, Req, Default) -> binary() | Default
+
+Name    :: binary()
+Req     :: cowboy_req:req()
+Default :: any()
+----
+
+Return the currently set response header value for the given HTTP header.
+
+== Arguments
+
+Name::
+
+Desired response HTTP header name as a binary.
+
+Req::
+
+The Req object.
+
+Default::
+
+Default value returned when the header is missing.
+
+== Return value
+
+The binary value for the given HTTP header name.
+
+== Changelog
+
+* *2.0*: Function introduced.
+
+== Examples
+
+.Get the response header with the given name
+[source,erlang]
+----
+HeaderValue = cowboy_req:resp_header(<<"x-test-header">>, Req).
+----
+
+.Get the response header with the given name and a default
+[source,erlang]
+----
+HeaderValue = cowboy_req:resp_header(<<"x-test-header">>, Req, <<"bar">>).
+----
+
+== See also
+
+link:man:cowboy_req(3)[cowboy_req(3)],
+link:man:cowboy_req:set_resp_headers(3)[cowboy_req:resp_headers(3)]
+link:man:cowboy_req:set_resp_header(3)[cowboy_req:set_resp_header(3)]