Fixed market order bug using Order.Price

[vslee]

Continuation of #615 with requested changes.
Original PR comment by @raymens

This change was required to make Marketorders possible on CoinbasePro.
Not sure what the other echanges allow and or require so I've kept it simple by just assuming the price is given.

I suppose it's also possible to add checks and fail in a nicer way, what's your take on it?

[raymens]

Thanks a lot, I had it on my list of to-dos. But I appreciate this!