Fix review suggestions

Author: chiranSachintha

bvm/ballerina-runtime/src/main/java/io/ballerina/runtime/internal/xml/XmlTreeBuilder.java

@@ -164,7 +164,7 @@ private void readPI(XMLStreamReader xmlStreamReader) {
 
     public static void addXmlDeclaration(XMLStreamReader reader, Deque<List<BXml>> siblingDeque) {
         String version = reader.getVersion();
-        String encoding = reader.getEncoding();
+        String encoding = reader.getCharacterEncodingScheme();
         boolean standalone = reader.isStandalone();
 
         StringBuilder piData = new StringBuilder();

tests/jballerina-unit-test/src/test/java/org/ballerinalang/test/types/xml/XMLLiteralTest.java

@@ -424,7 +424,7 @@ public void testXMLLiteralWithExtraDollarSigns() {
     public void testXMLToString() {
         BXml xml = XmlFactory.parse("<?xml version=\"1.0\" encoding=\"UTF-8\"?>" +
                 "<!DOCTYPE foo [<!ELEMENT foo ANY ><!ENTITY data \"Example\" >]><foo>&data;</foo>");
-        Assert.assertEquals(xml.toString(), "<?xml version=\"1.0\"?><foo>Example</foo>");
+        Assert.assertEquals(xml.toString(), "<?xml version=\"1.0\" encoding=\"UTF-8\"?><foo>Example</foo>");
     }
 
     @Test(dataProvider = "function-name-provider")

tests/jballerina-unit-test/src/test/java/org/ballerinalang/test/types/xml/XMLLiteralWithNamespacesTest.java

@@ -204,7 +204,7 @@ public void testXMLSerialize() {
     public void testXMLToString() {
         BXml xml = XmlFactory.parse("<?xml version=\"1.0\" encoding=\"UTF-8\"?>" +
                 "<!DOCTYPE foo [<!ELEMENT foo ANY ><!ENTITY data \"Example\" >]><foo>&data;</foo>");
-        Assert.assertEquals(xml.toString(), "<?xml version=\"1.0\"?><foo>Example</foo>");
+        Assert.assertEquals(xml.toString(), "<?xml version=\"1.0\" encoding=\"UTF-8\"?><foo>Example</foo>");
     }
 
     @Test (dataProvider = "xmlValueFunctions")