net/http: change a Del to delete

No need to fix case of "Host" string literal.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6278049

src/pkg/net/http/request.go

@@ -515,7 +515,7 @@ func ReadRequest(b *bufio.Reader) (req *Request, err error) {
 	if req.Host == "" {
 		req.Host = req.Header.get("Host")
 	}
-	req.Header.Del("Host")
+	delete(req.Header, "Host")
 
 	fixPragmaCacheControl(req.Header)