Reluctant as I am to edit an answer after it's been upvoted, some parts of this were not entirely correct and I've changed them. If you voted this up and now don't want to, feel free to retract your vote.
Dealing with an airline.
If an airline cancels a flight they have to give you a full refund. That's not just airline law, it's basic contract law. If you buy something, and the person you are buying from doesn't give it to you, they have to give you money back.
However airlines love to pretend that they don't have to do this. They send you a message saying that you have been given credit, and do their best to sound like that's the normal, or even the only, way it should work. They don't explicitly say you can't have the refund, they just try to steer you down the path of airline credit.
The way to handle this is to call them up and tell them that you want a full refund. Make it clear that you won't take anything else. Tell them you know you are legally entitled to one. They are unlikely to tell you outright that you can't have one (because that would be a lie and leave them open to a lawsuit) but they will try to persuade you to not take it.
You would also be entirely justified in getting the charge reversed on your credit card.
Dealing with Expedia
This question on the Law stack exchange talks about Expedia's US contract with its customers, which is both complicated and unclear, in the sense that it is both vague and hard to find. The contract fails to mention things like "Expedia credit", for example.
The contract states that you are paying Expedia some money (probably $45 in your case) to make a booking with the airline using the rest of the money you sent them. Since they made the booking you are probably not getting it back whatever happens. When the cancellation happens, the airline sends the refund money to Expedia, who sent it to them. The question is what should then happen to it. Contract law, and the Expedia contract, are vague on the matter.
On the face of it Expedia should refund the money to you. However Expedia - like the airlines - really, really wants to keep your money. They have very similar tactics in preferring to simply tell you that this is what has happened (they keep your money and give you 'credit') in the hope that you won't challenge this. Unfortunately it's much less clear that Expedia must refund your money if asked than that the airline must.
Having said that, experience seems to be that Expedia is not ready to push that point legally. There are plenty of stories of people who called Expedia and explicitly demanded their money back, and got it. Expedia probably isn't sure of its own legal ground and doesn't want to put it to the test, in case they are ordered to offer everybody a refund by default.
Calling Expedia and demanding a refund would seem to be the way to go. Some people also report success in getting credit card chargebacks, especially if that fails.
If I may editorialize a bit, all of the above would indicate to me that it is almost never worth actually booking through Expedia. Use them to find flights, but the flights are almost always available direct from the airline at a similar price, and can be booked directly without all the contract and refund hassle.