Add missing @discardableResult (#2338)

Source/SessionManager.swift

@@ -249,6 +249,7 @@ open class SessionManager {
     /// - parameter urlRequest: The URL request.
     ///
     /// - returns: The created `DataRequest`.
+    @discardableResult
     open func request(_ urlRequest: URLRequestConvertible) -> DataRequest {
         var originalRequest: URLRequest?