Spec the new output buffer argument for recv/recv_nonblock

* See #175.

library/socket/basicsocket/recv_spec.rb

@@ -76,4 +76,18 @@
     ScratchPad.recorded.should == "firstline\377"
   end
 
+  ruby_version_is "2.3" do
+    it "allows an output buffer as third argument" do
+      socket = TCPSocket.new('127.0.0.1', SocketSpecs.port)
+      socket.write("data")
+
+      client = @server.accept
+      buf = "foo"
+      client.recv(4, 0, buf)
+      client.close
+      buf.should == "data"
+
+      socket.close
+    end
+  end
 end

library/socket/shared/recv_nonblock.rb

@@ -31,6 +31,18 @@
       @s1.recv_nonblock(5).should == "aaa"
     end
 
+    ruby_version_is "2.3" do
+      it "allows an output buffer as third argument" do
+        @s1.bind(Socket.pack_sockaddr_in(SocketSpecs.port, "127.0.0.1"))
+        @s2.send("data", 0, @s1.getsockname)
+        IO.select([@s1], nil, nil, 2)
+
+        buf = "foo"
+        @s1.recv_nonblock(5, 0, buf)
+        buf.should == "data"
+      end
+    end
+
     it "does not block if there's no data available" do
       @s1.bind(Socket.pack_sockaddr_in(SocketSpecs.port, "127.0.0.1"))
       @s2.send("a", 0, @s1.getsockname)