fix: allow _patch to modify the entire base schema

[AshotN]

Summary

With this given schema

export const userSchema = Type.Object(
  {
    id: Type.Number(),
    username: Type.String(),
    isAdmin: Type.Boolean()
  },
  { $id: 'User', additionalProperties: false }
)

You may want a user to be able modify their own username, but obviously not set isAdmin to true whenever they want.

So you have a patch schema that only permits the username to be modified.

export const userPatchSchema = Type.Pick(userSchema, ['username'], {
  $id: 'UserPatch'
})

But the backend still retains the ability to modify any property by using underscore methods. _patch ignores all hooks and can set isAdmin to true

app.service('user')._patch(user.id, { isAdmin: true })

The issue is that currently Feathers throws a type error if you try to modify anything that isn't part of the userPatchSchema, even if you are using _patch. You get an error similar to this, Argument of type  { isAdmin: boolean; }  is not assignable to parameter of type  { username: string; } 

This is very frustrating to deal with, since the code is perfectly valid.

This PR attempts to address that type issue.

Other Information

I have tried my best in testing this, I ran the test script and all passed. I also tried this against my local feathers app and it seems to work correctly. I spent a decent bit of time on figuring out these whole 12 lines of changes. This was seemingly the simplest way of fixing the types, without breaking a lot of stuff

[daffl]

Thank you and sorry for the delay! This will go out with the next release 😄