Fixes #218 Discount Coupon and Shipping Cost

Fix calculation for Fixed Coupons with Shipping Cost
Add some unit tests. This just covers off the use case for the bug,
but need to add a lot more tests for other Discount Coupon test cases.
oops. bad line break
Fixes #218 - Discount Coupon and Shipping Cost

I have made all the other tests a bit more robust.
Also fixed the edge case Linda addressed where a Fixed Coupon + Free Shipping where include_shipping = true
would show an incorrect coupon value, albeit order totals were correct.

Longer term, for the discount coupon tests, it would probably be better if the test inputs and expected results
were defined in a hash map(array) otherwise we could end up with hundreds of combinations addressed by individual tests
Merge pull request #221 from zcwilt/discount-coupons

Fixes #218 Discount Coupon and Shipping Cost