Update schema.js

[joaoritter]

Summary

I've been trying to debug the following problem: when a custom typeKey and typePojoToMixed: false are passed as options to a schema, schema properties under the new typeKey that are a POJO get set as Mixed in the schema. This is incorrect, the type should be dynamically converted to a subschema, as per the line I have changed in this PR.
My change preserves the typeKey during the Object.assign, instead of using the default key of type.

Examples

const schema = mongoose.Schema({ a: { $type: { b: String, c: String } }}, { typeKey: '$type', typePojoToMixed: false });

// current buggy behavior: 
console.log({ schema }) // Schema { ..., paths: {  'a': [Mixed] } }   

// expected behavior: 
console.log({ schema }) // Schema { ..., paths: {  'a.b': [SchemaString], 'a.c': [SchemaString] } }   

[joaoritter]

I also noticed there are no test cases for setting a typeKey to a POJO. It's probably worth adding one if you intend to support it, or add to documentation if not.

[joaoritter]

This is blocking me right now, I'd appreciate a quick acknowledgement of this PR. I understand if it takes a bit to merge, but I just want to make sure im not missing something simple. I could attempt other work arounds if this is in fact a bug and it'll take a while to merge a fix.

[joaoritter]

I dug up this commit where this line was last changed. It would be useful to add a test with a typeKey here.
b4b61d3

Thanks!

[vkarpov15]

Thanks 👍

[joaoritter]

100%, thank YOU for all your work on this project.