net: make .write() throw on bad input

Passing a non-buffer or non-string argument to Socket.prototype.write triggered
an assert:

  Assertion failed: (Buffer::HasInstance(args[0])), function Write,
  file ../src/stream_wrap.cc, line 289.

Fixes #2532.

lib/net.js

@@ -424,8 +424,10 @@ Socket.prototype.write = function(data, arg1, arg2) {
   }
 
   // Change strings to buffers. SLOW
-  if (typeof data == 'string') {
+  if (typeof data === 'string') {
     data = new Buffer(data, encoding);
+  } else if (!Buffer.isBuffer(data)) {
+    throw new TypeError("First argument must be a buffer or a string.");
   }
 
   this.bytesWritten += data.length;

test/simple/test-net-connect-buffer.js

@@ -63,6 +63,25 @@ tcp.listen(common.PORT, function() {
 
   assert.equal('opening', socket.readyState);
 
+  // Make sure that anything besides a buffer or a string throws.
+  [ null,
+    true,
+    false,
+    undefined,
+    1,
+    1.0,
+    1 / 0,
+    +Infinity
+    -Infinity,
+    [],
+    {}
+  ].forEach(function(v) {
+    function f() {
+      socket.write(v);
+    }
+    assert.throws(f, TypeError);
+  });
+
   // Write a string that contains a multi-byte character sequence to test that
   // `bytesWritten` is incremented with the # of bytes, not # of characters.
   var a = "L'État, c'est ";