[json] proposals include true and false for boolean type regardless of enum

[castwide]

This issue was discovered in rubyide/vscode-ruby#295.

Add a configuration contribution to an extension's package.json that accepts both boolean and string types, but give it an enum that doesn't include one of the boolean options:

"ruby.codeCompletion": {
  "type": ["boolean", "string"],
  "enum": [false, "rcodetools"],
  "default": false,
  "description": "Method to use for code completion. Use `false` if another extension provides this feature."
}

When editing the configuration in User Settings, the completion items include true despite the enum:

[sandy081]

@aeschli I think it is related to JSON