Refactor engine. Fix infinite recursion in configuration

Source/SocketEngine.swift

@@ -359,36 +359,38 @@ public final class SocketEngine : NSObject, NSURLSessionDelegate, SocketEnginePo
     private func handleOpen(openData: String) {
         do {
             let json = try openData.toNSDictionary()
-            
-            if let sid = json["sid"] as? String {
-                let upgradeWs: Bool
-
-                self.sid = sid
-                connected = true
-
-                if let upgrades = json["upgrades"] as? [String] {
-                    upgradeWs = upgrades.contains("websocket")
-                } else {
-                    upgradeWs = false
-                }
-
-                if let pingInterval = json["pingInterval"] as? Double, pingTimeout = json["pingTimeout"] as? Double {
-                    self.pingInterval = pingInterval / 1000.0
-                    self.pingTimeout = pingTimeout / 1000.0
-                }
-
-                if !forcePolling && !forceWebsockets && upgradeWs {
-                    createWebsocketAndConnect()
-                }
-                
-                sendPing()
-                
-                if !forceWebsockets {
-                    doPoll()
-                }
-                
-                client?.engineDidOpen("Connect")
+            guard let sid = json[sid] as? String else {
+                client?.engineDidError("Open packet contained no sid")
+                return
             }
+            
+            let upgradeWs: Bool
+            
+            self.sid = sid
+            connected = true
+            
+            if let upgrades = json["upgrades"] as? [String] {
+                upgradeWs = upgrades.contains("websocket")
+            } else {
+                upgradeWs = false
+            }
+            
+            if let pingInterval = json["pingInterval"] as? Double, pingTimeout = json["pingTimeout"] as? Double {
+                self.pingInterval = pingInterval / 1000.0
+                self.pingTimeout = pingTimeout / 1000.0
+            }
+            
+            if !forcePolling && !forceWebsockets && upgradeWs {
+                createWebsocketAndConnect()
+            }
+            
+            sendPing()
+            
+            if !forceWebsockets {
+                doPoll()
+            }
+            
+            client?.engineDidOpen("Connect")
         } catch {
             didError("Error parsing open packet")
         }

Source/SocketIOClientConfiguration.swift

@@ -97,7 +97,7 @@ public struct SocketIOClientConfiguration : ArrayLiteralConvertible, CollectionT
     
     @warn_unused_result
     public func prefixThrough(position: Index) -> SubSequence {
-        return prefixThrough(position)
+        return backingArray.prefixThrough(position)
     }
 
     @warn_unused_result