Fix case of coerced zero numeric subtraction.

[smudge]

Before:

This works as expected:

Money.new(0) - Money.new(100)
=> #<Money fractional:-100 currency:USD>

But this does not:

0 - Money.new(100)
=> #<Money fractional:100 currency:USD>

A common case for the above would be when summing a list and subtracing a value from the sum. For instance, this works:

my_list = [Money.new(-10), Money.new(10)]
my_list.sum - Money.new(100)
=> #<Money fractional:-100 currency:USD>

But if you happen to end up with an empty list and forget to handle it:

my_list = []
my_list.sum - Money.new(100)
=> #<Money fractional:100 currency:USD>

Uh oh! That's a bug!

Rather than requiring the developer to know to provide a default Money.empty value to .sum (which would fix the latter issue, but not the former one), this PR addresses the underlying issue of commutativeness being assumed during coercion even on subtraction. In reality, subtraction is not commutative, even if all we care about is the zero numeric case.

After:

Once we apply the correct order on subtraction with a zero CoercedNumeric, we get a negative monetary amount:

0 - Money.new(100)
=> #<Money fractional:-100 currency:USD>

my_list = []
my_list.sum - Money.new(100)
=> #<Money fractional:-100 currency:USD>

[coveralls]

Coverage decreased (-0.6%) to 99.322% when pulling c7f0d4e on smudge:fix-zero-minus-money into 0a157b6 on RubyMoney:master.

[coveralls]

Coverage decreased (-0.6%) to 99.322% when pulling c7f0d4e on smudge:fix-zero-minus-money into 0a157b6 on RubyMoney:master.

[coveralls]

Coverage decreased (-0.6%) to 99.322% when pulling c7f0d4e on smudge:fix-zero-minus-money into 0a157b6 on RubyMoney:master.

[coveralls]

Coverage decreased (-0.6%) to 99.322% when pulling c7f0d4e on smudge:fix-zero-minus-money into 0a157b6 on RubyMoney:master.

[coveralls]

Coverage decreased (-0.6%) to 99.322% when pulling c7f0d4e on smudge:fix-zero-minus-money into 0a157b6 on RubyMoney:master.

[antstorm]

Makes perfect sense, great PR, thank you!