Make duplicate attribute check O(1), detect duplicates in xmlns mode.
Upstream: https://github.com/isaacs/sax-js/pull/296

diff --git a/sax.js b/sax.js
--- a/sax.js
+++ b/sax.js
@@ -63,6 +63,7 @@
     parser.strictEntities = parser.opt.strictEntities
     parser.ENTITIES = parser.strictEntities ? Object.create(sax.XML_ENTITIES) : Object.create(sax.ENTITIES)
     parser.attribList = []
+    parser.attribNames = Object.create(null)
 
     // namespaces form a prototype chain.
     // it always points at the current tag,
@@ -688,6 +689,7 @@
       tag.ns = parent.ns
     }
     parser.attribList.length = 0
+    parser.attribNames = Object.create(null)
     emitNode(parser, 'onopentagstart', tag)
   }
 
@@ -711,11 +713,11 @@
       parser.attribName = parser.attribName[parser.looseCase]()
     }
 
-    if (parser.attribList.indexOf(parser.attribName) !== -1 ||
-      parser.tag.attributes.hasOwnProperty(parser.attribName)) {
+    if (parser.attribNames[parser.attribName]) {
       parser.attribName = parser.attribValue = ''
       return
     }
+    parser.attribNames[parser.attribName] = true
 
     if (parser.opt.xmlns) {
       var qn = qname(parser.attribName, true)
@@ -835,6 +837,7 @@
     }
     parser.attribName = parser.attribValue = ''
     parser.attribList.length = 0
+    parser.attribNames = Object.create(null)
   }
 
   function closeTag (parser) {
@@ -905,6 +908,7 @@
     if (t === 0) parser.closedRoot = true
     parser.tagName = parser.attribValue = parser.attribName = ''
     parser.attribList.length = 0
+    parser.attribNames = Object.create(null)
     parser.state = S.TEXT
   }