title: immutable JS - immutable deep merge is overriding the object
body: <p>I am using Immutable JS in my node project to hold the very huge blob of map in the memory. Every key has an object as a value. Example:</p>
  
  <pre><code>const obj1 = {
    "foo.bar": {
       valid: ['baz']
    }
  }
  
  const obj2 = {
    "foo.bar": {
       key1: 'abc',
       key2: [1, 2, 3],
       valid: ['baz', 'bar', 'foo']
    }
  }
  </code></pre>
  
  <p>expected Output for <code>const obj3 = Immutable.Map(obj2).mergeDeep(Immutable.Map(obj1))</code> is</p>
  
  <pre><code>"foo.bar": {
       key1: 'abc',
       key2: [1, 2, 3],
  
       valid: ['baz'] // here
  
    }
  </code></pre>
  
  <p>What i get is</p>
  
  <pre><code>"foo.bar": {
       valid: ['baz']
   }
  </code></pre>
  
  <p>I am using ImmutbaleJs 3.8.1.</p>