Bug: Check blocked words against alphabet case-insensitively

[4kimov]

Short version: New bug was found in the spec repo. It was filtering blocklist against the alphabet incorrectly in the constructor, and the issue showed up when using lowercase blocklist in uppercase-only alphabet.

More detailed version: See pull request #7 discussion.

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New test was added to the spec: sqids/sqids-spec@02d6116
Code to fix the issue: sqids/sqids-spec@c9c8b28

[jesperoman]

#6 should do it!