dgram: don't assert on send('string')

Raise a TypeError when the argument to send() or sendto() is anything
but a Buffer.

Fixes the following assertion:

  $ node -e 'require("dgram").createSocket("udp4").send("BAM")'
  node: ../../src/udp_wrap.cc:220: static v8::Handle<v8::Value>
  node::UDPWrap::DoSend(const v8::Arguments&, int): Assertion
  `Buffer::HasInstance(args[0])' failed.
  Aborted (core dumped)

Fixes #4496.
object_wrap: add missing HandleScope

by Sven Panne's suggestion, see [0] for details.

[0]: https://code.google.com/p/v8/issues/detail?id=2468