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/ripple/protocol/impl/Feature.cpp

@@ -130,6 +130,7 @@ detail::supportedAmendments ()
         "DeletableAccounts",
         "fixQualityUpperBound",
         "RequireFullyCanonicalSig",
+        "fix1781",
     };
     return supported;
 }
@@ -189,5 +190,6 @@ uint256 const fixPayChanRecipientOwnerDir = *getRegisteredFeature("fixPayChanRec
 uint256 const featureDeletableAccounts = *getRegisteredFeature("DeletableAccounts");
 uint256 const fixQualityUpperBound = *getRegisteredFeature("fixQualityUpperBound");
 uint256 const featureRequireFullyCanonicalSig = *getRegisteredFeature("RequireFullyCanonicalSig");
+uint256 const fix1781 = *getRegisteredFeature("fix1781");
 
 } // ripple