Fix merging objects with prototypes

[aleksandryackovlev]

The merge function in /source/merge.js uses Object.assign method.
The issue is that this method copies only an object's own properties to the resulting object.

It means we've got an error from https://github.com/nodejs/node/blob/master/lib/_http_client.js#L135 when we pass httpsAgent or httpAgent as an option to any method of the client.

For example, the following code will throw th an error.

await client.putFileContents(filePath, fileBuffer, { httpsAgent: new https.Agent({ rejectUnauthorized: true })});

To fix this issue we have to not only copy its own properties of the original object but to copy its prototype too.

[perry-mitchell]

Hmm.. very interesting. I made a suggestion, to improve the cloning process, but I simply wonder if a helper like the following would be better:

function isPureObject(input) {
    if (input === null || typeof input === "undefined") return false;
    return Object.getPrototypeOf(input).isPrototypeOf(Object);
}

We could use this to not merge objects that are non-object types (classes, like HttpsAgent).

We'd need to also test for Array.isArray() as [] doesn't satisfy isPureObject of course.

[aleksandryackovlev]

Maybe it's going to be better to combine both approaches.
It seems like in lodash.merge they use both of them.
https://github.com/lodash/lodash/blob/master/.internal/baseMergeDeep.js

[perry-mitchell]

Thanks @aleksandryackovlev! This is great. Will merge shortly.