Fix MockSession fileUrl

Issue is: now fileUrl generate random file urls for the same request,
cause the digest of request body is random.

There is a breaking change in swift Foundation's JSONEncoder,
sortedKeys needed now.

For detail, check here: https://developer.apple.com/forums/thread/735895

Sources/EOSIO/API/Client.swift

@@ -251,6 +251,7 @@ extension Client {
         encoder.dataEncodingStrategy = self.dataEncoder
         encoder.dateEncodingStrategy = self.dateEncoder
         encoder.keyEncodingStrategy = .convertToSnakeCase
+        encoder.outputFormatting = .sortedKeys
         return encoder
     }
 }