Document _fields is case sensitive

Bug description

The documentation around Searching user identities  doesn't list that is case sensitive

https://backstage.forgerock.com/docs/am/6.5/maintenance-guide/#rest-api-read-identity

Use the _fields query string parameter to restrict the list of attributes returned. This parameter takes a comma-separated list of JSON object fields to include in the result:

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How to reproduce the issue

If you hit an end point such as 

/openam/json/realms/root/realms/CustomerRealm/users/UserName?_fields=UiD

If uid is a different case such as "uid" then this will fail to return a result. If you specify the uid correctly, such as below, then this will succeed. 

/openam/json/realms/root/realms/CustomerRealm/users/UserName?_fields=uid

Expected behaviour

OPENAM-4136 listed this as a change in AM 12 

Current behaviour

The _fields arguments are case sensitive

Work around

Use the proper case.