case sensitive adjustments

[patrick-ogrady]

Viper has opted to not support case-sensitive keys (https://github.com/spf13/viper#does-viper-support-case-sensitive-keys):

Does Viper support case sensitive keys?
tl;dr: No.

Viper merges configuration from various sources, many of which are either case insensitive or uses different casing than the rest of the sources (eg. env vars). In order to provide the best experience when using multiple sources, the decision has been made to make all keys case insensitive.

There has been several attempts to implement case sensitivity, but unfortunately it's not that trivial. We might take a stab at implementing it in Viper v2, but despite the initial noise, it does not seem to be requested that much.

You can vote for case sensitivity by filling out this feedback form: https://forms.gle/R6faU74qPRPAzchZ9

This PR applies changes described in a candidate PR (spf13#635) to optionally unlock this functionality.