Improve update schemas

[eleftherioszisis]

• Do not allow to silently ignore excluded values. Let the user know they are updating forbidden attributes.
• Merge default keys in the make_update_schema instead of using them as defaults to avoid skipping the default keys by mistake

[GianlucaFicarelli]

We can also consider to return an error if any unknown field is passed in the update and/or create models, by specifying extra='forbid' in the schema.
Any of these changes could break some clients if they pass unsupported fields.