Make completion handler mandatory on multiple queries

**WARNING: Breaking change.** Multiple queries are just aggregated search queries, so it does not make any sense to ignore the results. Furthermore, it contradicts the convention adopted in the rest of the code.

Author: Clément Le Provost

Source/Client.swift

@@ -240,7 +240,7 @@ import Foundation
     /// - parameter completionHandler: Completion handler to be notified of the request's outcome.
     /// - returns: A cancellable operation.
     ///
-    @objc public func multipleQueries(queries: [IndexQuery], strategy: String?, completionHandler: CompletionHandler? = nil) -> NSOperation {
+    @objc public func multipleQueries(queries: [IndexQuery], strategy: String?, completionHandler: CompletionHandler) -> NSOperation {
         // IMPLEMENTATION NOTE: Objective-C bridgeable alternative.
         var path = "1/indexes/*/queries"
         if strategy != nil {
@@ -265,7 +265,7 @@ import Foundation
     /// - parameter completionHandler: Completion handler to be notified of the request's outcome.
     /// - returns: A cancellable operation.
     ///
-    public func multipleQueries(queries: [IndexQuery], strategy: MultipleQueriesStrategy? = nil, completionHandler: CompletionHandler? = nil) -> NSOperation {
+    public func multipleQueries(queries: [IndexQuery], strategy: MultipleQueriesStrategy? = nil, completionHandler: CompletionHandler) -> NSOperation {
         // IMPLEMENTATION NOTE: Not Objective-C bridgeable because of enum.
         return multipleQueries(queries, strategy: strategy?.rawValue, completionHandler: completionHandler)
     }