Bug 11680: (follow-up) fix unexpected tax rate changes on edit

Bug 11680: (follow-up) fix unexpected tax rate changes on edit

Follow-up to fix similar issue on vendor edit.

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

Test plan for this follow-up:

1) insert some tax rates with trailing zeroes in gist
system preference (e.g: '0|0.080|0.12|0.20|0.23')
2) add some vendors, choosing 8.0% 12.0% 20.0% 23.0%
as gst rate
3) try to modify them
4) note that vendors 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 vendors

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>