Fix variable name in config_context_{list, _hash}

[ayosec]

Fix a typo in the variable name used in the code for both config_context_list and config_context_hash.

Without the fix, when I try to use these methods in strict mode, I get the following error:

Mixlib::Config::UnknownConfigOptionError:
 Reading unsupported config value symbol.

The included tests can reproduce the error if you revert the changes in lib/mixlib/config.rb

[thommay]

Duplicate of #57

[thommay]

Thanks for the PR. Given that we've got two identical ones I'm going to take the earlier one. Sorry for the delay in getting this merged.

[thommay]

The fix is released in 2.2.8