@burcidi, @feruze-- I don't think the meaning has changed that much. It's still mainly means a man that comes to the help of another. I've never heard anyone refer to a woman as a knight in shining armor, have you?
The most famous use of the idiom has to be when a man saves a woman from danger and protects her. So it has a romantic aspect to it too in its modern use.
When my boyfriend helps me with something, small or big, I tell him he's my knight in shining armor. I think he likes that a lot.