Check XRP endpoints for circular paths (RIPD-1781):

The payment engine restricts payment paths so two steps do not input the
same Currency/Issuer or output the same Currency/Issuer. This check was
skipped when the path started or ended with XRP. An example of a path
that was incorrectly accepted was: XRP -> //USD -> //XRP -> EUR

This patch enables the path loop check for paths that start or end with
XRP.

src/test/app/PayStrand_test.cpp

@@ -703,7 +703,7 @@ struct PayStrand_test : public beast::unit_test::suite
                     alice,
                     /*deliver*/ xrpIssue(),
                     /*limitQuality*/ boost::none,
-                    /*sendMaxIssue*/ xrpIssue(),
+                    /*sendMaxIssue*/ EUR.issue(),
                     path,
                     true,
                     false,