Bug in integer arithmetic meaning `col1 - col2` becomes `col2 - col2`

[ryanvarley]

If you subtract two numeric columns from each other piccolo will subtract it from itself.

e.g.

class MyTable(Table):
    integer = Integer(null=True)
    other_integer = Integer(null=True)

print(MyTable.integer - MyTable.other_integer) produces other_integer - other_integer

As far as I can see there are no tests cases for column on column arithmetic.

PR Demonstrating the issue #1260

[dantownsend]

@ryanvarley Ah yes, I see what you mean. Good catch!

[dantownsend]

Resolved by #1260