Fix config_context_list/hash in strict mode

[elyscape]

Without this, config_context_list and config_context_hash are completely broken in strict mode:

irb(main):001:0> require 'mixlib/config'
=> true
irb(main):002:0> class Config
irb(main):003:1>   extend Mixlib::Config
irb(main):004:1>   config_strict_mode true
irb(main):005:1>   config_context_list :tests, :test do
irb(main):006:2*     default :x, 5
irb(main):007:2>   end
irb(main):008:1> end
Mixlib::Config::UnknownConfigOptionError: Reading unsupported config value symbol.

This fixes that.

[thommay]

Looks great, thanks!

[thommay]

Thanks for your contribution, I've released this as 2.2.8.