added NICK already registered test

test/server.js

@@ -37,6 +37,22 @@ describe('IRC server', function() {
                 done("Destroyed socket without answering")
             }
 
+            server.emit("connection", mockedSock)
+            mockedSock.emit('data', Buffer.from("NICK some_nick\r\n", "ascii"))
+        })
+    }),
+        describe("NICK already registered", function () {
+            it("should handle a NICK command -> NICK some_nickname", function (done) {
+                const server = IRCServer.create()
+                let mockedSock = new EventEmitter()
+                mockedSock.write = function (data) {
+                    assert.equal(data.toString("ascii"), "433")
+                    done()
+                }
+                mockedSock.destroy = function () {
+                    done("Destroyed socket without answering")
+                }
+
                 server.emit("connection", mockedSock)
                 mockedSock.emit('data', Buffer.from("NICK some_nick\r\n", "ascii"))
             })