Make Payload case-insensitive (Fixes #108)

[russ]

Problem

Fixes #108.

Crystal's JSON parsing is case-sensitive, so a payload like {"Command": "Subscribe"} failed two separate ways:

1. Field name — Payload.from_json raised Missing JSON attribute: command because the key was Command, not command.
2. Command value — even once parsed, Connection#receive dispatched on payload.command == "subscribe", so a Subscribe value never matched.

Fix

• Payload.from_json normalizes the top-level keys (command / identifier / data) to lowercase before deserializing. Only the top-level keys are touched — the nested values (channel names and the identifier/data blobs) are left exactly as sent, since those stay case-sensitive (e.g. the channel name still has to match the registered channel class). Invalid payloads still raise JSON::SerializableError / JSON::ParseException as before, so the Cable::Handler error path is unchanged.
• Connection#receive compares the command case-insensitively (payload.command.downcase).

Tests

• A Payload unit spec parsing mixed-case top-level keys ("Command", "Identifier", "DATA") and confirming command, channel, data, and action all resolve.
• An end-to-end #receive spec sending {"Command": "Subscribe", "Identifier": ...} and asserting it confirms the subscription.

Both specs fail on master with the exact Missing JSON attribute: command from the issue, and pass with this change. Full suite is green locally on Crystal 1.10.0 and current stable.

Note

The nightly CI job is continue-on-error: true and currently fails repo-wide because ameba won't compile against Crystal nightly (undefined method 'next_string_array_token') — it fails during shards install, before any project code, so it's unrelated to this change.

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