Measure stretch doesn't work on multimeasure rests

MuseScore 187353b7d2

Create a score with multimeasure rests
Select the multi measure rest measure
Layout -> Add more stretch

Expected result: the measure is enlarged
Actual result: the measure doesn't change.

It's a regression compare to MuseScore 1.3.

First reported here : http://musescore.org/en/node/25205