Updating python dicts
===

If I have dictionary A:

    { 'name': 'Jack',
      'age': 24,
      'friends': ['Bob', 'Alice', 'Karen'],
    }

and I have dictionary B: 

    { 'name': 'Jack',
      'favorite_color': 'blue',
      'friends': ['Daren'],
    }

How can I combine them so that the result only has fields that were in dictionary A and where the two have the same field update the value in A based on the value in B. And when the field is an array append to it instead of replacing it. 

So the resulting dictionary C should be:

    { 'name': 'Jack',
      'age': 24,
      'friends': ['Bob', 'Alice', 'Karen', 'Daren'],
    }

