Assure PaymentRequest.id is a UUID (closes #588)

Given that all UAs use a UUID for an .id, we make this a must.

Author: Marcos Cáceres

index.html

@@ -560,12 +560,12 @@ 
           li>
           <li>Establish the request's id:
             <ol>
-              <li>If <var>detailsvar>.<a data-lt=
-              "PaymentDetailsInit.id">ida> is missing, add an <a data-lt=
-              "PaymentDetailsInit.id">ida> member to <var>detailsvar> and
-              set its value to string that uniquely identifies this payment
-              request. It is RECOMMENDED that the string be a <abbr title=
-              "Universally Unique Identifier">UUIDabbr> [[!RFC4122]].
+              <li data-tests="payment-request-id-attribute.https.html">If <var>
+                detailsvar>.<a data-lt="PaymentDetailsInit.id">ida> is
+                missing, add an <a data-lt="PaymentDetailsInit.id">ida>
+                member to <var>detailsvar> and set its value to a
+                <abbr title="Universally Unique Identifier">UUIDabbr>
+                [[!RFC4122]].
               li>
             ol>
           li>