Fix for .returns object inequality - #79

Fixes #36

This fixes the issue of ticket 36. It didn't have anything to do with promises, but with how the arguments equality for a mock with .return was performed.
So if we mocked a method where one of the arguments was an object or class, unless the object had the same reference, areArgumentsEqual function returned false.

const a = { value: 1 }
const b = a
a === b // true

const c = { value: 1 }
a === c // false

When this ocurred, the mocked method / property, instead of returning the expected value, it returned context.proxy, which had all sort of side effects, one of them being that sometimes an error was thrown with the message Cannot convert object to primitive value.

I made a little change on the proxies, they are based now on a dummy SubstituteJS class. I'm not sure yet if this will help with debugging when something goes wrong, but by doing that, and adding the getOwnPropertyDescriptor handler, instead of getting the arguments not matching error with the generic [[Function]], it will write [[SubstituteJS]]

Error {
    message: `Expected 1 call to the method storeUpdates with arguments.....
    ....All calls received to method storeUpdates:␊
    -> call with arguments [[Function]]`,
  }
// ------
VS
// ------
Error {
    message: `Expected 1 call to the method storeUpdates with arguments....
    .....All calls received to method storeUpdates:␊
    -> call with arguments [[SubstituteJS]]`,
  }