Dry::Configurable::TestInterface::reset_config won't reset nested settings

[spectator]

Hi,

This PR adds falling test to demonstrate the issue with Dry::Configurable::TestInterface::rest_config that won't reset nested settings.

[GustavoCaso]

@spectator I have been looking and it looks when using configure with nested keys it will not store the default value instead it store the config object with the value from the configure block.

[#<Dry::Configurable::Config::Value:0x007fc6099b8120 @name=:dsn, @value=nil, @processor=#<Proc:0x007fc60b0e2c10@/Users/morty/dev/gems/dry-configurable/lib/dry/configurable/config.rb:5 (lambda)>>, #<Dry::Configurable::Config::Value:0x007fc609a23510 @name=:pool, @value=#<#<Class:0x007fc60aa230f0>:0x007fc60aa21fc0 @config={:size=>5}>, @processor=#<Proc:0x007fc60b0e2c10@/Users/morty/dev/gems/dry-configurable/lib/dry/configurable/config.rb:5 (lambda)>>]

I do not have much experience with this library, but I think maybe there will need to have some changes regarding the internal structure and how they handle nested attributes.

@AMHOL I'll be happy to give a hand with this one. I was thinking instead of returning the actual proc when calling config

#<#<Class:0x007fc60b2f2708>:0x007fc60b2f23e8 @config={:dsn=>"sqlite:memory", :pool=>#<#<Class:0x007fc60aa230f0>:0x007fc60aa21fc0 @config={:size=>5}>}>

We could return an OpenStruct which will allow the user to access with the intended interface:
klass.config.pool.size. That wey we could store the actual default value for the nested attributes.