Bug 16129: Remove URL::Encode dependency

 Bug 16129: Remove URL::Encode dependency

This patch makes the PayPal integration feature independent from the
URL::Encode library, which is absent in some supported distributions.

It uses the URI package which is already a Koha dependency.

To test:
- Apply the patch
- Notice there are no deps for URL::Encode
- Follow the steps from the original patch
=> SUCCESS: It works as expected
- Sign-off :-D

Note: I deleted the line in which $amount_to_pay was url-encoded, because that's
one of the things query_form does (and the variable is only used as a parameter to it).

Sponsored-by: ByWater Solutions
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>