5.0.0

Breaking

• POST /register (/signup, /users) returns 409 Conflict instead of
  400 for an email that is already registered. The body is unchanged
  (bare JSON string "Email already exists"). Clients checking for 400 on
  this case need to check for 409. Other register validation errors
  (missing fields, invalid email, short password) stay 400.
• POST /books rejects a duplicate ISBN with 409 Conflict and the bare
  JSON string "Book with ISBN <isbn> already exists"; no second book is
  stored. In 4.x such a request returned 201 and silently stored a duplicate,
  so clients that create the same ISBN twice now get an error. This restores
  the 3.x behavior: since 4.0 the id is a UUID instead of the ISBN, so the
  implicit duplicate-id check of json-server (which surfaced as a 500 in 3.x)
  was gone.