private Map<String, SignerConfig> mergeSignerMap(JsonIndex<SignerConfigJsonHelper, SignerConfig>[] defaults,
                                                     JsonIndex<SignerConfigJsonHelper, SignerConfig>[] overrides,
                                                     String theme) {
        Map<String, SignerConfig> map = buildSignerMap(defaults, theme);
        Map<String, SignerConfig> mapOverride = buildSignerMap(overrides, theme);
        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