Fix RangeError exception when merging too many labels (#5936)

Fix "RangeError: Maximum call stack size exceeded" exception when calling `Array.push.apply` with too many items (>125000).

src/scales/scale.time.js

@@ -554,7 +554,9 @@ module.exports = function() {
 							datasets[i][j] = timestamp;
 						}
 					} else {
-						timestamps.push.apply(timestamps, labels);
+						for (j = 0, jlen = labels.length; j < jlen; ++j) {
+							timestamps.push(labels[j]);
+						}
 						datasets[i] = labels.slice(0);
 					}
 				} else {