Auto-identify the XML namespace Issue 22

While the XML namespace may be declared it does not have to be in order
to use xml namespaced attributes. This code will automatically recognize
xml namespaced attributes, even if the namespace is not explicitly
declared.

Author: jbrayfaithlife

src/AngleSharp.Xml.Tests/Parser/XmlParsing.cs

@@ -146,5 +146,13 @@ public void ParseInvalidXmlShouldNotThrowWhenSuppressingErrors_Issue14()
                 parser.ParseDocument(source);
             });
         }
+
+        [Test]
+        public async Task XmlPrefixedAttributesShouldLocateXmlNamespaceWithoutDeclaration()
+        {
+            var document = @"<xml xml:lang=""en""></xml>".ToXmlDocument();
+            var root = document.DocumentElement;
+            Assert.AreEqual(NamespaceNames.XmlUri, root.Attributes.Single().NamespaceUri);
+        }
     }
 }

src/AngleSharp.Xml/Parser/XmlDomBuilder.cs

@@ -431,7 +431,11 @@ private Attr CreateAttribute(String name, String value)
                 var prefix = name.Substring(0, colon);
                 var ns = NamespaceNames.XmlNsUri;
 
-                if (!prefix.Is(NamespaceNames.XmlNsPrefix))
+                if (prefix.Is(NamespaceNames.XmlPrefix))
+                {
+                    ns = NamespaceNames.XmlUri;
+                }
+                else if (!prefix.Is(NamespaceNames.XmlNsPrefix))
                 {
                     ns = CurrentNode.LookupNamespaceUri(prefix);
                 }