[PR #1489] Updated RequestRetrier completion to be escaping.

Allows the `RequestRetrier` to perform async work before executing the
`completion` closure.

Source/Request.swift

@@ -52,7 +52,7 @@ public protocol RequestRetrier {
     /// - parameter request:    The request that failed due to the encountered error.
     /// - parameter error:      The error encountered when executing the request.
     /// - parameter completion: The completion closure to be executed when retry decision has been determined.
-    func should(_ manager: SessionManager, retry request: Request, with error: Error, completion: RequestRetryCompletion)
+    func should(_ manager: SessionManager, retry request: Request, with error: Error, completion: @escaping RequestRetryCompletion)
 }
 
 // MARK: -