Set keys case sensitive

[gbunt]

Kudos to @knadh

Add viper.SetKeysCaseSensitive() to disable automatic key lowercasing.

YAML, TOML, and JSON dictate keys to be case-sensitive. Viper's default
behaviour of lowercasing the keys for key insensitivity is incompatible
with these standards and has the side effect of making it difficult for
use cases such as case sensitive API credentials in configuration.
For eg: MyApiKey=MySecret (in TOML).

See #131, #260, #293, #371, #373

This commit adds a global function viper.SetKeysCaseSensitive() that
enables this behaviour to be turned off, after which, all keys, irrespective
of nesting, retain their cases. This respects all configuration operations
including getting, setting, and merging.

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1 out of 2 committers have signed the CLA.

✅ knadh
❌ gbunt
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