Bug#25946 Namespace not include for xsi usage within --xml output with null/nil values

Fix: adding namespace reference into "mysql --xml" output,
to make it work similary to "mysqldump --xml".

client/mysql.cc

@@ -2529,7 +2529,8 @@ print_table_data_xml(MYSQL_RES *result)
 
   tee_fputs("<?xml version=\"1.0\"?>\n\n<resultset statement=\"", PAGER);
   xmlencode_print(glob_buffer.ptr(), (int)strlen(glob_buffer.ptr()));
-  tee_fputs("\">", PAGER);
+  tee_fputs("\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\">",
+            PAGER);
 
   fields = mysql_fetch_fields(result);
   while ((cur = mysql_fetch_row(result)))

mysql-test/r/client_xml.result

@@ -7,7 +7,7 @@ insert into t1 values (1, 2, 'a&b a<b a>b');
 <?xml version="1.0"?>
 
 <resultset statement="select * from t1
-">
+" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <row>
 	<field name="a&amp;b">1</field>
 	<field name="a&lt;b">2</field>
@@ -34,7 +34,7 @@ insert into t1 values (1, 2, 'a&b a<b a>b');
 <?xml version="1.0"?>
 
 <resultset statement="select count(*) from t1
-">
+" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <row>
 	<field name="count(*)">1</field>
   </row>
@@ -42,7 +42,7 @@ insert into t1 values (1, 2, 'a&b a<b a>b');
 <?xml version="1.0"?>
 
 <resultset statement="select 1 &lt; 2 from dual
-">
+" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <row>
 	<field name="1 &lt; 2">1</field>
   </row>
@@ -50,7 +50,7 @@ insert into t1 values (1, 2, 'a&b a<b a>b');
 <?xml version="1.0"?>
 
 <resultset statement="select 1 &gt; 2 from dual
-">
+" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <row>
 	<field name="1 &gt; 2">0</field>
   </row>
@@ -58,7 +58,7 @@ insert into t1 values (1, 2, 'a&b a<b a>b');
 <?xml version="1.0"?>
 
 <resultset statement="select 1 &amp; 3 from dual
-">
+" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <row>
 	<field name="1 &amp; 3">1</field>
   </row>
@@ -66,7 +66,7 @@ insert into t1 values (1, 2, 'a&b a<b a>b');
 <?xml version="1.0"?>
 
 <resultset statement="select null from dual
-">
+" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <row>
 	<field name="NULL" xsi:nil="true" />
   </row>