Let me clarity. Don't expect anything with this one. You took the first step towards better things with women, in general. You didn't do a good job establishing your boundaries and your degree of tolerance for bull $hit from the start, so you were forced to do it this way. YOu know better for next time.
If she respects you and/or fears doing goofy stuff like that, because you are a catch and doesn't want to ruin her chances, #1 shes wouldn't do what she did, #2, you would be able to get away with a "Kool, have fun" type of response. It is clear she had no fear of losing you, as she admited she made plans with another guy. When they are not interested, they will make up scenarios like that one, and throw it at you and see how you respond. If you are the type of guy she can walk all over and is too accomodating, she will take your kindness as weakness. A man worth his time would never tolerate this kind of BS.
The context of the situation and dynamics of the relationship will dictate how non-chalant your approach can be. If she had very high interest, she would have made up a better excuse, therefore giving you teh chance to spit your, "cool, have fun" type of response. Also, if she had high interest, she would cancel other plans, and would most definitely not make "new" plans. She would also counter-offer right away, and sincerely beg you for a chance to make it up to you.
If you have plenty of women around you and many options, then you wouldn't even bother worrying about this one, and could get away with saying "cool, have a good time", whenever a broad tells you that she rather hang out with another guy. Do not second guess your response. You reacted this, now you stick to it and move on.