[crystal][client] some fix and minor improvement

[YusukeHosonuma]

This PR fix follows.

• Remove redundant include JSON::Serializable.
• Fix syntax-error in Hash literal.
• Fix shardName in shard.yml.
• Add explicitly require "json"

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@wing328

[wing328]

@YusukeHosonuma thanks for the PR, which looks good to me.

[YusukeHosonuma]

@wing328
Thanks for review!
If I find some improvement, I'll another PR.