fix datarace in input apache plugin

closes #1384

plugins/inputs/apache/apache_test.go

@@ -36,7 +36,8 @@ func TestHTTPApache(t *testing.T) {
 	defer ts.Close()
 
 	a := Apache{
-		Urls: []string{ts.URL},
+		// Fetch it 2 times to catch possible data races.
+		Urls: []string{ts.URL, ts.URL},
 	}
 
 	var acc testutil.Accumulator