Fix merging of Buffer objects

[sds]

Previously, if you attempted to build/create an object whose properties contained Buffers, they would be converted from the native Buffer type into an array (and thus creating an incorrect value). This is caused by the use of lodash's merge implementation (see lodash/lodash#2964).

The recommended solution from the lodash project is to define a merge customizer to specifically handle this situation, which is what we do here.

[pfletcherhill]

Having the same problem with Uint8Array. Can we include that too and get this merged?

[pangolingo]

@rakeshpetit and I are going to see if we can write some unit tests to go along with this PR, and verify that this fix is both needed and effective.

[pangolingo]

Thanks for this PR!

Could we add a test to prevent future regressions? Here's what I wrote when testing this change:

// lib/__tests__/merge-params.test.ts

it('does not convert array-like objects to arrays when merging', () => {
    type User = {
      a: ArrayBuffer;
      b: Buffer;
      c: Uint8Array;
      d: Float32Array;
      e: Int8Array;
      f: DataView;
      g: string[];
      nested: {
        a: ArrayBuffer;
        b: Buffer;
        c: Uint8Array;
        d: Float32Array;
        e: Int8Array;
        f: DataView;
        g: string[];
      };
    };
    const userFactory = Factory.define<User>(() => ({
      a: new ArrayBuffer(5),
      b: Buffer.from('hello'),
      c: new Uint8Array(5),
      d: new Float32Array(5),
      e: new Int8Array(5),
      f: new DataView(new ArrayBuffer(5)),
      g: ['hello'],
      nested: {
        a: new ArrayBuffer(5),
        b: Buffer.from('hello'),
        c: new Uint8Array(5),
        d: new Float32Array(5),
        e: new Int8Array(5),
        f: new DataView(new ArrayBuffer(5)),
        g: ['hello'],
      },
    }));
    const user = userFactory.build({
      a: new ArrayBuffer(5),
      b: Buffer.from('hello'),
      c: new Uint8Array(5),
      d: new Float32Array(5),
      e: new Int8Array(5),
      f: new DataView(new ArrayBuffer(5)),
      g: ['hello'],
      nested: {
        a: new ArrayBuffer(5),
        b: Buffer.from('hello'),
        c: new Uint8Array(5),
        d: new Float32Array(5),
        e: new Int8Array(5),
        f: new DataView(new ArrayBuffer(5)),
        g: ['hello'],
      },
    });
    expect(user.a).toBeInstanceOf(ArrayBuffer);
    expect(user.b).toBeInstanceOf(Buffer);
    expect(user.c).toBeInstanceOf(Uint8Array);
    expect(user.d).toBeInstanceOf(Float32Array);
    expect(user.e).toBeInstanceOf(Int8Array);
    expect(user.f).toBeInstanceOf(DataView);
    expect(Array.isArray(user.g)).toBe(true);

    expect(user.nested.a).toBeInstanceOf(ArrayBuffer);
    expect(user.nested.b).toBeInstanceOf(Buffer);
    expect(user.nested.c).toBeInstanceOf(Uint8Array);
    expect(user.nested.d).toBeInstanceOf(Float32Array);
    expect(user.nested.e).toBeInstanceOf(Int8Array);
    expect(user.nested.f).toBeInstanceOf(DataView);
    expect(Array.isArray(user.nested.g)).toBe(true);
  });

[pangolingo]

Great solution. To help with future maintenance, I'd love to add a comment here

Suggested change

	if (Array.isArray(srcVal) || ArrayBuffer.isView(srcVal)) {
	return srcVal;
	if (Array.isArray(srcVal) || ArrayBuffer.isView(srcVal)) {
	// don't try to merge arrays or subclasses of TypedArray
	// ArrayBuffer.isView() matches Buffer, Uint8Array, Float32Array, etc.
	return srcVal;