[flow] enums with underscore produce an invalid object key

Describe the bug

When you use _ as a value on a graphql enum, the flow plugin fails to generate a valid object definition for such enum. It creates an object with a missing key (take a look below for examples).

To Reproduce Use the following enum on a Graphql schema:

enum Gender {
    M
    F
    _
}

1. My GraphQL schema:

schema {
  query: Query
  mutation: Mutation
}

type Mutation {
  # Pay schedules
  addUser(input: addUserInput!): User
}

type Query {
  node(id: ID!): Node
  viewer: Viewer
}

interface Node {
  id: ID!
}

type Viewer implements Node {
  id: ID!
  user(id: ID!): User!
}

input addUserInput {
    name: String!
    email: String!
}

type User {
    id: ID!
    username: String!
    email: String!
}

enum Gender {
    M
    F
    _
}```

2. My GraphQL operations:

I don't know what this is
```graphql
# Put your operations here

1. My codegen.yml config file:

overwrite: true
schema: "schema.graphql"
documents: 
  - "./document.graphql"
  - "./*.js"
generates:
  generated/graphql.js:
    plugins:
      - "flow"
      - "flow-operations"
  ./graphql.schema.json:
    plugins:
      - "introspection"

Expected behavior I expect it to use ‘_’ or at least ‘’ as property name, instead it is producing the following invalid JS object definition:

export const GenderValues = Object.freeze({
  M: 'M',
  F: 'F',
  : '_'
});

Additional context

Reproduction: https://codesandbox.io/s/enumfailure-l7j7b?file=/document.js

[Urigo]

I don’t think there is a real reason cc @dotansimha

[dotansimha]

By the way, I noticed the namingConvention docs are under certain plug-ins documentation. Is there a reason it is not on the general section?

No real reason, just because the configuration is generated from the plugins code, so it sits next to that.

I merged your PR and the new documentation page should be live in a few minutes. Thank you!