plugin/rewrite: use request.Request and other cleanups (#1920)

This was done anyway, but only deep in the functions, just do this
everywhere; allows for shorter code and request.Request allows for
caching as well.

Cleanups, make it more Go like.
* remove unneeded switches
* remove testdir (why was this there??)
* simplify the logic
* remove unneeded variables
* put short functions on a single line
* fix documentation.
* spin off wire funcs in wire.go, make them functions.

Signed-off-by: Miek Gieben <miek@miek.nl>

request/request.go

@@ -59,6 +59,15 @@ func (r *Request) IP() string {
 	return r.ip
 }
 
+// LocalIP gets the (local) IP address of server handling the request.
+func (r *Request) LocalIP() string {
+	ip, _, err := net.SplitHostPort(r.W.LocalAddr().String())
+	if err != nil {
+		return r.W.LocalAddr().String()
+	}
+	return ip
+}
+
 // Port gets the (remote) port of the client making the request.
 func (r *Request) Port() string {
 	if r.port != "" {