Some JSON files do not contain a signingName.

This commit checks if it exists, and if not
uses `endpointPrefix` as before.

Author: Ómar Kjartan Yasin

lib/aws_codegen/json_service.ex

@@ -41,10 +41,14 @@ defmodule AWS.CodeGen.JSONService do
 
   defp build_context(language, module_name, api_spec, doc_spec) do
     actions = collect_actions(language, api_spec, doc_spec)
+    signing_name = case api_spec["metadata"]["signingName"] do
+     :nil -> api_spec["metadata"]["endpointPrefix"];
+     sn   -> sn
+    end
     %Service{actions: actions,
              docstring: Docstring.format(language, doc_spec["service"]),
              endpoint_prefix: api_spec["metadata"]["endpointPrefix"],
-             signing_name: api_spec["metadata"]["signingName"],
+             signing_name: signing_name,
              json_version: api_spec["metadata"]["jsonVersion"],
              module_name: module_name,
              protocol: api_spec["metadata"]["json"],