fixed update.assign for deep object merges

[AleksandrZhukov]

• common js Object.assign doesn't have ability to merge objects deeply so added mergeDeep for these needs
• note: arrays will be replaced with new one

[akuzko]

@AleksandrZhukov sorry for long reply.
a couple issues with this PR:

Initially project was built in pure ES5 with no traspiling, so the code at the moment you opened this PR had to be ES5-compatible. transpiling was added in v1.7.3, so now this has to be just rebased.

It is intended for update.assign to behave exactly like Object.assign, but preserving object immutability. It's hard to show a simple use-case where this might be important, since this is related with deep object nesting, but consider something like this:

const initialTodos = {
  settings: {
    can: {}
  },
  items: []
};

// at some point of time we have something like
data = {
  todos: {
    settings: {
      can: {edit: true}
    },
    items: [todo1],
    valueWeWantToKeep: true
  }
}

// then later on we want to drop portion of `todos` data with

const nextData = update.assign(data, 'todos', initialTodos);

with deep-assign behavior, this code won't drop edit: true value from can object, which also can be pretty unexpected.

what I would actually suggest is to add deepAssign or assignDeep, or preferrably, merge helper with the code that you have implemented, and that will be a great addition to the package