XML: Fix handling elements w/o namespace inside elements w/ ns.

The problem occurred when namespace prefixes in Clarck notation were
stripped (default behavior) and affected both the standard etree and
lxml modes.

After the fix elements w/o ns inside elements w/ ns get `xmlns=""`
when using the standard etree. When using lxml, namespace prefixes are
preserved correctly. These fixes resolve #1784.

atest/robot/standard_libraries/xml/namespaces.robot

@@ -20,5 +20,8 @@ Saved XML is semantically same as original
 Saved XML has same content as original but only default namespaces
     Check Test Case    ${TESTNAME}
 
+Element without namepace inside element with namespace
+    Check Test Case    ${TESTNAME}
+
 Attribute namespaces are not handled
     Check Test Case    ${TESTNAME}

atest/robot/standard_libraries/xml/namespaces_with_lxml.robot

@@ -20,5 +20,8 @@ Saved XML is semantically same as original
 Saved XML has same namespaces as original
     Check Test Case    ${TESTNAME}
 
+Element without namepace inside element with namespace
+    Check Test Case    ${TESTNAME}
+
 Attribute namespaces are not handled
     Check Test Case    ${TESTNAME}

atest/testdata/standard_libraries/xml/namespaces.robot

@@ -64,6 +64,13 @@ Saved XML has same content as original but only default namespaces
     ...   ${INDENT}<back>back in default</back>
     ...   </test>
 
+Element without namepace inside element with namespace
+    Save XML    ${NO NS IN NS}    ${OUTPUT}
+    Elements Should Be Equal    ${NO NS IN NS}    ${OUTPUT}
+    Saved XML Should Equal    ${NO NS IN NS}
+    ...    <root xmlns="uri"><no xmlns=""><yes xmlns="uri"><no xmlns="">.</no></yes></no></root>
+    Element Text Should Be    ${NO NS IN NS}    .    xpath=no/yes/no
+
 Attribute namespaces are not handled
     ${elem} =    Parse XML    ${ATTR NS}
     Test Attribute Namespace Parsing    ${elem}

atest/testdata/standard_libraries/xml/namespaces_with_lxml.robot

@@ -49,6 +49,12 @@ Saved XML is semantically same as original
 Saved XML has same namespaces as original
     Saved XML Should Equal File    ${NS}    ${NS}
 
+Element without namepace inside element with namespace
+    Save XML    ${NO NS IN NS}    ${OUTPUT}
+    Elements Should Be Equal    ${NO NS IN NS}    ${OUTPUT}
+    Saved XML Should Equal    ${NO NS IN NS}    ${NO NS IN NS}
+    Element Text Should Be    ${NO NS IN NS}    .    xpath=no/yes/no
+
 Attribute namespaces are not handled
     ${elem} =    Parse XML    ${ATTR NS}
     Test Attribute Namespace Parsing With lxml    ${elem}

atest/testdata/standard_libraries/xml/xml_resource.robot

@@ -6,7 +6,7 @@ Library            String
 *** Variables ***
 ${TEST} =          ${CURDIR}/test.xml
 ${NS} =            ${CURDIR}/namespaces.xml
-${DEFAULT NS} =    ${CURDIR}/default_namespaces.xml
+${NO NS IN NS} =   <ns:root xmlns:ns="uri"><no><ns:yes><no>.</no></ns:yes></no></ns:root>
 ${SIMPLE} =        <root><child id="1">text</child><c2><gc /></c2></root>
 ${ATTR NS} =       <root id="1" p:id="2" xmlns:p="xxx" />
 ${OUTPUT} =        %{TEMPDIR}/xmllib.xml

src/robot/libraries/XML.py

@@ -1324,6 +1324,9 @@ def strip(self, elem, current_ns=None):
             if ns != current_ns:
                 elem.attrib['xmlns'] = ns
                 current_ns = ns
+        elif current_ns:
+            elem.attrib['xmlns'] = ''
+            current_ns = None
         for child in elem:
             self.strip(child, current_ns)