When reading configuration from sources with case-sensitive keys,
such as YAML, TOML, and JSON, a user may wish to preserve the case
of keys that appear in maps. For example, consider when the value
of a setting is a map with string keys that are case-sensitive.
Ideally, if the value is not going to be indexed by a Viper lookup
key, then the map value should be treated as an opaque value by
Viper, and its keys should not be modified. See #1014
Viper's default behaviour is that keys are case-sensitive, and this
behavior is implemented by converting all keys to lower-case. For
users that wish to preserve the case of keys, this commit introduces
an Option `CaseSensitiveKeys()` that can be used to configure Viper
to use case-sensitive keys. When CaseSensitiveKeys is enabled, all
keys retain the original case, and lookups become case-sensitive
(except for lookups of values bound to environment variables).
The behavior of Viper could become hard to understand if a user
could change the CaseSensitiveKeys setting after values have been
stored. For this reason, the setting may only be set when creating
a Viper instance, and it cannot be set on the "global" Viper.
