kubectl: Handle splitting host:port when no port is present

In the proxy filter, if listening on port 80 or 443 there will
be no port present in the Host: HTTP header. Just use the entire
thing in these cases, and don't reject requests when no port
is present in the Host: HTTP header.

pkg/kubectl/proxy_server_test.go

@@ -309,3 +309,16 @@ func TestPathHandling(t *testing.T) {
 		}()
 	}
 }
+
+func TestExtractHost(t *testing.T) {
+	fixtures := map[string]string{
+		"localhost:8085": "localhost",
+		"marmalade":      "marmalade",
+	}
+	for header, expected := range fixtures {
+		host := extractHost(header)
+		if host != expected {
+			t.Fatalf("%s != %s", host, expected)
+		}
+	}
+}