[Issue #1100] Simplified `User-Agent` header logic leveraging more type-safety.

Source/Manager.swift

@@ -56,10 +56,10 @@ public class Manager {
         // User-Agent Header; see https://tools.ietf.org/html/rfc7231#section-5.5.3
         let userAgent: String = {
             if let info = NSBundle.mainBundle().infoDictionary {
-                let executable: AnyObject = info[kCFBundleExecutableKey as String] ?? "Unknown"
-                let bundle: AnyObject = info[kCFBundleIdentifierKey as String] ?? "Unknown"
-                let version: AnyObject = info[kCFBundleVersionKey as String] ?? "Unknown"
-                let os: AnyObject = NSProcessInfo.processInfo().operatingSystemVersionString ?? "Unknown"
+                let executable = info[kCFBundleExecutableKey as String] as? String ?? "Unknown"
+                let bundle = info[kCFBundleIdentifierKey as String] as? String ?? "Unknown"
+                let version = info[kCFBundleVersionKey as String] as? String ?? "Unknown"
+                let os = NSProcessInfo.processInfo().operatingSystemVersionString
 
                 var mutableUserAgent = NSMutableString(string: "\(executable)/\(bundle) (\(version); OS \(os))") as CFMutableString
                 let transform = NSString(string: "Any-Latin; Latin-ASCII; [:^ASCII:] Remove") as CFString