enveloped-signature - only rm immediate child sigs

212200b changed the xpath to match all signature nodes descended from
the specified local node. This is incorrect; the relevant spec says that
only immediate signature elements should be removed

This commit corrects the behaviour simply by changing the xpath to only
match immediate children

Fixes #135 , where further discussion can be found

Author: LoneRifle

lib/enveloped-signature.js

@@ -6,7 +6,7 @@ function EnvelopedSignature() {
 }
 
 EnvelopedSignature.prototype.process = function (node) {
-  var signature = xpath(node, ".//*[local-name(.)='Signature' and namespace-uri(.)='http://www.w3.org/2000/09/xmldsig#']")[0];  
+  var signature = xpath(node, "./*[local-name(.)='Signature' and namespace-uri(.)='http://www.w3.org/2000/09/xmldsig#']")[0];  
   if (signature) signature.parentNode.removeChild(signature);
   return node;
 };