Extend schema does not extend properties, but overrides them

[jmav]

Following example will override title attribute because prop in NEW_SCHEMA is overriding one from SCHEMA_BASE.

To Reproduce

const SCHEMA_BASE = S.object()
  .prop(
    'reason',
    S.string()
      .title('Reason 1')
  )

const NEW_SCHEMA = S.object()
  .extend(SCHEMA_BASE)
  .prop(
    'reason',
    S.string()
      .minLength(1)
  )
  

Expected behavior

Props should be extended with with only changed attributes.

Environment

• node version: 10
• fastify version: >=2.0.0
• os: Mac