First thing out of the tanks mouth: Healz you know how to dispel right?
first thing out of my mouth is: You know how to jump right?
The tank's question was a valid one and you acted like a prick.
You could rephase his question as "healer do you have the dispel talent?" and you took it as a stab at your e-honor or something.
Just yesterday I had this cute little paladin who didn't spec into dispel magic and told us nothing as we got to the last boss on H VP (actually he said 'go go go' when the tank asked if everyone was ready). You see, I can't read minds and it would be nice to know that the fight is now super hard nightmare mode and I'm not allowed to make a mistake. We wiped due to people getting clinged through the fight and the healer not being able to dispel it. Once the healer finally admited not specing into magic dispel we just got wiser and avoided all clings and finished the run.
So no, asking if the healer knows how to dispel is not rude.
This is a perfect example of how people could just take 5 seconds more to think before they type to have a much better pug experience. Coming in as a new healer and finding yourself at the last boss in any dungeon is ALWAYS a bad sign. Then, to have someone questions whether you know how to play your spec just adds to the uneasiness, and people get defensive. If the tank had said "We're having trouble with Static Cling, do you have Dispel in your spec?" the healer probably would have reacted better.
Now, the healer isn't off the hook. The tank probably didn't mean anything by his question, and remember to give people the benefit of the doubt first. A good response could have been "I can Dispel, but it takes away from healing. If people time a jump or hop to right before the cast they won't get hit with it."
If either people are still acting like 8 year olds, wait for the start of the boss and /leave – at least you tried.
TL:DR - Be excellent to each other...and party on dudes!