Propagate http header callback from websocket back to client.

Source/SocketIO/Client/SocketIOClientSpec.swift

@@ -344,4 +344,16 @@ public enum SocketClientEvent : String {
     /// }
     /// ```
     case statusChange
+
+    /// Emitted when when upgrading the http connection to a websocket connection.
+    ///
+    /// Usage:
+    ///
+    /// ```swift
+    /// socket.on(clientEvent: .websocketUpgrade) {data, ack in
+    ///     let headers = (data as [Any])[0]
+    ///     // Some header logic
+    /// }
+    /// ```
+    case websocketUpgrade
 }

Source/SocketIO/Engine/SocketEngine.swift

@@ -313,6 +313,12 @@ open class SocketEngine : NSObject, URLSessionDelegate, SocketEnginePollable, So
             this.parseEngineMessage(message)
         }
 
+        ws?.onHttpResponseHeaders = {[weak self] headers in
+            guard let this = self else { return }
+
+            this.client?.engineDidWebsocketUpgrade(headers: headers)
+        }
+
         ws?.connect()
     }
 
@@ -666,7 +672,9 @@ open class SocketEngine : NSObject, URLSessionDelegate, SocketEnginePollable, So
         connected = false
         polling = true
 
-        if let reason = error?.localizedDescription {
+        if let error = error as? WSError {
+            didError(reason: "\(error.message). code=\(error.code), type=\(error.type)")
+        } else if let reason = error?.localizedDescription {
             didError(reason: reason)
         } else {
             client?.engineDidClose(reason: "Socket Disconnected")

Source/SocketIO/Engine/SocketEngineClient.swift

@@ -59,4 +59,9 @@ import Foundation
     ///
     /// - parameter data: The data the engine received.
     func parseEngineBinaryData(_ data: Data)
+
+    /// Called when when upgrading the http connection to a websocket connection.
+    ///
+    /// - parameter headers: The http headers.
+    func engineDidWebsocketUpgrade(headers: [String: String])
 }

Source/SocketIO/Manager/SocketManager.swift

@@ -377,6 +377,18 @@ open class SocketManager : NSObject, SocketManagerSpec, SocketParsable, SocketDa
         }
     }
 
+    /// Called when when upgrading the http connection to a websocket connection.
+    ///
+    /// - parameter headers: The http headers.
+    open func engineDidWebsocketUpgrade(headers: [String: String]) {
+        handleQueue.async {
+            self._engineDidWebsocketUpgrade(headers: headers)
+        }
+    }
+     private func _engineDidWebsocketUpgrade(headers: [String: String]) {
+        emitAll(clientEvent: .websocketUpgrade, data: [headers])
+    }
+
     /// Called when the engine has a message that must be parsed.
     ///
     /// - parameter msg: The message that needs parsing.