Excess workspace group when loading summed workspaces

[mantid-builder]

Original Reporter: Arturs Bekasovs

# Clear ADS
mtd.clear()

# Load 2 summed workspaces
ws = Load(Filename='MUSR15190+15191')

# ADS:
# * MUSR00015190_MUSR00015191 - shouldn't be here
#   - MUSR00015190_MUSR00015191_1
#   - MUSR00015190_MUSR00015191_2
# * ws
#   - MUSR00015190_MUSR00015191_1
#   - MUSR00015190_MUSR00015191_2

This works when summing using '-' as well. When trying to delete excess group I get this:

DeleteWorkspace('MUSR00015190_MUSR00015191')

# ADS:
# * MUSR00015190_MUSR00015191 - empty group

[mantid-builder]

This issue was originally trac ticket 8315