Fix NICK command behaviour to RFC

according to the RFC the correct response to the NICK command then it's
changing a nickname is to send a NICK command to the user from the
original nickname.

This patch changes the behaviour and removes a duplicate declearation of
the function.

src/user.js

@@ -1,12 +1,15 @@
 function User(socket, authenticatedDefault) {
   this.registered = false
   this.authenticated = authenticatedDefault
-  this.nickname = ""
+  this.nickname = undefined
   this.connection = socket
   this.realname = ""
   this.username = ""
 
   this.setNickname = function(nickname) {
+    if (this.nickname !== undefined) {
+      this.sendRaw(`:${this.nickname} NICK ${nickname}`)
+    }
     this.nickname = nickname
   }
 
@@ -26,12 +29,6 @@ function User(socket, authenticatedDefault) {
   this.getAddress = function() {
     return this.connection.address().address
   }
-    this.setNickname = function (nickname) {
-        this.nickname = nickname
-    }
-    this.setRealName = function (realname) {
-        this.realname = realname
-    }
 
     this.closeConnection = function () {
         this.connection.destroy()

test/user.js

@@ -62,4 +62,23 @@ describe('User', function () {
             user.closeConnection()
         })
     })
+
+    describe('#setNickname(nickname)', function() {
+        it('should not answer on inital setting of nickname', function () {
+            let mockedSock = {}
+            const user = new User(mockedSock, true)
+            user.setNickname("some_nick")
+            assert.equal(user.getNickname(), "some_nick")
+        })
+
+        it('should answer with NICK message from original nick on rename', function (done) {
+            let mockedSock = {write: function (data) {
+                assert.equal(data, ":some_nick NICK changed_nick\r\n")
+                done()
+            }}
+            const user = new User(mockedSock, true)
+            user.setNickname("some_nick")
+            user.setNickname("changed_nick")
+        })
+    })
 })