Add mergedConfig support

[omidmt]

I use this plugin in my plugin as helper, so needed to provision the config of cookie plugin by default. The good way of providing default config values for plugin is 'plugin-config', but they are appear as mergedConfig not config. So I just added a check to this object before deciding to use default value of cookie plugin it-self.

[benlucchesi]

Hi,

Thanks for the contribution. I'll get this integrated shortly.

-ben

[double16]

mergedConfig looks like a Grails 2.x thing, can you confirm? If it is 3.x I will merge into https://github.com/double16/grails-cookie-session/tree/release/4.0.0