Plus isn't summing total proton charge correctly if RHSWorkspace=Output

[martyngigg]

From Aziz (I adapted his script to give the one below to illuminate the exact issue). This is affecting his reduction scripts

file1 = 'HRP39187.raw'
file2 = 'HRP39191.RAW'

Load(Filename=file1,OutputWorkspace='total_sum',LoadLogFiles='0')
print 'Charge from file1',mtd['total_sum'].getRun().getProtonCharge()
Load(Filename=file2,OutputWorkspace='__temp',LoadLogFiles='0')
print 'Charge from file2',str(mtd['__temp'].getRun().getProtonCharge())

# Sum into separate workspace
Plus(LHSWorkspace="total_sum",RHSWorkspace="__temp", OutputWorkspace="separate_sum")
print 'Charge from sum into separate workspace',str(mtd['separate_sum'].getRun().getProtonCharge())

# Sum with accumulator on LHS
Plus(LHSWorkspace="total_sum",RHSWorkspace="__temp", OutputWorkspace="total_sum")
print 'Charge from sum with accumulator on LHS',str(mtd['total_sum'].getRun().getProtonCharge())

# Sum with accumulator on RHS
# Reload total sum
Load(Filename=file1,OutputWorkspace='total_sum',LoadLogFiles='0')
Plus(LHSWorkspace="__temp",RHSWorkspace="total_sum", OutputWorkspace="total_sum")
print 'Charge from sum with accumulator on RHS',str(mtd['total_sum'].getRun().getProtonCharge())

Script output:

Charge from file1 225.018066406
Charge from file2 175.025421143
Charge from sum into separate workspace 400.043487549
Charge from sum with accumulator on LHS 400.043487549
Charge from sum with accumulator on RHS 350.050842285

[martyngigg]

This issue was originally trac ticket 4561