Bug 11680: fix case where tax rate changes unexpectedly on editing an order

Bug 11680: fix case where tax rate changes unexpectedly on editing an order

If the tax rates in Acquisitions -> gist system preference are entered
with trailing zeroes, given order tax rate value may not be correctly
handled on order edit.

To test:

1) insert some tax rates with trailing zeroes in gist
system preference (e.g: '0|0.080|0.12|0.20|0.23')
2) place some new orders, choosing 8.0% 12.0% 20.0% 23.0%
as gst rate
3) try to modify them
4) note that orders with 12.0% and 23.0% tax rates are
preserving previously choosen rates on edit, while
the ones with 8.0% and 20.0% do not (they are defaulting
to the first defined tax rate)
5) apply the patch
6) repeat 2) and 3)
7) all tax rates configured in system prefrence shall now
behave properly while editing orders

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
All tests pass

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Confirmed the problem and that this patch fixes it.
Problem also exists for editing the default tax rate of a vendor.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 471215dc46e5a65360a3bde53666f62f809f0861)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>