azure login token caching should support case insensitive user name

The change was made around the "find" logic to do case insensitive match on the 'userId' file, which is the simplest code change i can find.
I considered normalizing the user name to lowercase before adding to the cache. But after thought a whole I decided not to do it for the following reason:
   a. Multiple routine will need to be updated, no only 'find', but also 'add' & 'remove'
   b. adal-node appears casing aware, though on some cases we know it can deal with case insensitive, because of that, I prefer not to tweak the casing myself which might land us to unknown troubles in future,
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