private <C extends Builder<T>, T> Map<String, T> merge(JsonIndex<C, T>[] defaults, JsonIndex<C, T>[] overrides) {
        Map<String, T> map = buildMap(defaults);
        Map<String, T> mapOverride = buildMap(overrides);
        map.putAll(mapOverride);
        return Collections.unmodifiableMap(map);
    }
Returns an immutable map by merging the override signer configuration into the default signer
configuration for the given theme.

@param defaults
           default signer configuration
@param override
           signer configurations overrides
@param theme
           used for message logging. eg region, service, region+service
aws-java-sdk-core/src/main/java/com/amazonaws/internal/config/InternalConfig.java