buffer: fix Buffer::Copy regression from 00b4b7b

If the end argument is omitted or not a number, make it default to
the end of the buffer, not zero.

Ideally, it should not matter what it defaults to because the JS shim
in lib/buffer.js should handle that but there are still several places
in node.js core that secrete SlowBuffers, hence Buffer::Copy() gets
called without going through Buffer.prototype.copy() first.

src/node_buffer.cc

@@ -403,7 +403,8 @@ Handle<Value> Buffer::Copy(const Arguments &args) {
   size_t target_length = Buffer::Length(target);
   size_t target_start = args[1]->Uint32Value();
   size_t source_start = args[2]->Uint32Value();
-  size_t source_end = args[3]->Uint32Value();
+  size_t source_end = args[3]->IsUint32() ? args[3]->Uint32Value()
+                                          : source->length_;
 
   if (source_end < source_start) {
     return ThrowRangeError("sourceEnd < sourceStart");