Draining fuel is slow when the rocket's internal fuel buffer is filled beyond the destination's fuel requirement

If a rocket is set to a faraway destination, filled with fuel, then set to a closer location, pumps are unable to drain the rocket fuel buffer at the expected rate.

Steps to reproduce
1. Build a rocket and set it's destination to a location in a different solar system
2. Fill the rocket with fuel.
3. Set the rocket's destination to Nauvis Orbit. The extra rocket fuel will be hidden.
4. Attempt to drain the fuel directly into a fluid tank. This will be much slower than expected; I estimate around 1 fluid unit per second.
5. Set the rocket's destination back to the original target. Now that the fuel GUI shows the correct amount of fuel, the pump will drain the fuel correctly.