[core] Fix XmlEnvironment namespace: Use default namespace (not element's)

Assume the element
<foo:bar xmlns='namespace' xmlns:foo='foo-namespace'/>

then the <bar/> element's namespace is 'foo-namespace', but the
default namespace is 'namespace'. And this is the namespace that
scopes into inner elements.

smack-core/src/main/java/org/jivesoftware/smack/packet/XmlEnvironment.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2019 Florian Schmaus
+ * Copyright 2019-2021 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -133,7 +133,7 @@ public class XmlEnvironment {
     }
 
     public static XmlEnvironment from(XmlPullParser parser, XmlEnvironment outerXmlEnvironment) {
-        String namespace = parser.getNamespace();
+        String namespace = parser.getDefaultNamespace();
         String xmlLang = ParserUtils.getXmlLang(parser);
         return new XmlEnvironment(namespace, xmlLang, outerXmlEnvironment);
     }

smack-core/src/test/java/org/jivesoftware/smack/xml/XmlPullParserTest.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2019 Florian Schmaus
+ * Copyright 2019-2021 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,9 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 import java.io.IOException;
 import java.io.Reader;
 import java.io.StringReader;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Function;
 import java.util.stream.Stream;
 
 import javax.xml.XMLConstants;
@@ -207,6 +210,34 @@ public class XmlPullParserTest {
         }
     }
 
+    @ParameterizedTest
+    @EnumSource(SmackTestUtil.XmlPullParserKind.class)
+    public void testGetNamespace(SmackTestUtil.XmlPullParserKind parserKind)
+                    throws XmlPullParserException, IOException {
+        String xml = "<foo:bar xmlns='namespace' xmlns:foo='foo-namespace'/>";
+        XmlPullParser parser = SmackTestUtil.getParserFor(xml, parserKind);
+
+        String parsedName = parser.getName();
+        assertEquals("bar", parsedName);
+
+        String parsedPrefix = parser.getPrefix();
+        assertEquals("foo", parsedPrefix);
+
+        String parsedPrefixNamespace = parser.getNamespace(parsedPrefix);
+        assertEquals("foo-namespace", parsedPrefixNamespace);
+
+        String parsedNamespace = parser.getNamespace();
+        assertEquals("foo-namespace", parsedNamespace);
+
+        List<Function<XmlPullParser, String>>  defaultNamespaceRetrievers = new ArrayList<>();
+        defaultNamespaceRetrievers.add(p -> p.getNamespace(null));
+        defaultNamespaceRetrievers.add(p -> p.getDefaultNamespace());
+        for (Function<XmlPullParser, String> defaultNamespaceRetriever : defaultNamespaceRetrievers) {
+            String defaultNamespace = defaultNamespaceRetriever.apply(parser);
+            assertEquals("namespace", defaultNamespace);
+        }
+    }
+
     private static void assertAttributeHolds(XmlPullParser parser, int attributeIndex, String expectedLocalpart,
                     String expectedPrefix, String expectedNamespace) {
         QName qname = parser.getAttributeQName(attributeIndex);

smack-xmlparser/src/main/java/org/jivesoftware/smack/xml/XmlPullParser.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2019 Florian Schmaus.
+ * Copyright 2019-2021 Florian Schmaus.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -75,6 +75,10 @@ public interface XmlPullParser {
 
     String getNamespace(String prefix);
 
+    default String getDefaultNamespace() {
+        return getNamespace(null);
+    }
+
     int getDepth();
 
     String getPositionDescription();