Revert "Removed mention of Buffer in byteSourceToUint8Array"

This reverts commit f00e06c9.

Although Node Buffers are not covered by the tests, it seems that there
are users relying on them, so we need to add back in support for them.

This fixes issue #4359.

js/binary/utils.js

@@ -971,6 +971,10 @@ jspb.utils.byteSourceToUint8Array = function(data) {
     return /** @type {!Uint8Array} */(new Uint8Array(data));
   }
 
+  if (data.constructor === Buffer) {
+    return /** @type {!Uint8Array} */(new Uint8Array(data));
+  }
+
   if (data.constructor === Array) {
     data = /** @type {!Array<number>} */(data);
     return /** @type {!Uint8Array} */(new Uint8Array(data));