Allow 1000 as a WebSocket close code

[brianquinlan]

Fixes #1203

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[catapop84]

Hello, You should allow a range between 1000-4999 according to official doc: https://developer.mozilla.org/en-US/docs/Web/API/CloseEvent/code and https://www.rfc-editor.org/rfc/rfc6455.html#section-7.1.5