[Cart Operation] Cannot Checkout as Guest

[pmclain]

Preconditions (*)

1. Storefront with at least one product and guest checkout enabled.

Steps to reproduce (*)

Using only graphql...

1. Create cart for guest
2. Add product to cart
3. Add cart shipping address
4. Set cart shipping method
5. Set cart billing address
6. Set cart payment method
7. Place order

Or execute tests added in #614

Expected result (*)

1. placeOrder mutation should return order increment id

Actual result (*)

1. Guest checkout currently fails with the error: GraphQL response contains errors: Guest email for cart is missing. Please enter`

I don't see path for adding the guest email to a quote via graphql.

[pmclain]

A possible mutation for setting the email:

type Mutation {
  setGuestCartEmail(input: SetGuestCartEmailInput): SetGuestCartEmailOutput
}

type SetGuestCartEmailInput {
  cart_id: String!
  email: String!
}

type SetGuestCartEmailOutput {
  cart: Cart!
}

[atwixfirster]

Related PRs:

• [Checkout coverage] setGuestEmailOnCart mutation #564
• [Checkout coverage] Place order concept #404

CC: @rogyar