only escape characters xml need to be escaped

[leavesster]

as I mentioned at #73 , escape /n/t is not necessary. These words is allowed to be saved at XML。

I think it's quite strange that while html.EscapeString replace the XML escape characters, xml.EscapeText escape more characters than XML allow.

After merge this pr

/t/n will show it origin characters

when developer read a xml

<?xml version="1.0" encoding="utf-8"?>
	<class_list xml:space="preserve">
		<student>
			<name> Robert </name>
			<grade>A+</grade>

		</student>
	</class_list>
`

doc OutputXML will give developer as it was, and it comply with xml specification

before this pr was merge , OutputXML will give human unreadable like this:
<?xml version=\"1.0\" encoding=\"utf-8\"?><class_list xml:space=\"preserve\">&#xA;&#x9;&#x9;<student>&#xA;&#x9;&#x9;&#x9;<name> Robert </name>&#xA;&#x9;&#x9;&#x9;<grade>A+</grade>&#xA;&#xA;&#x9;&#x9;</student>&#xA;&#x9;</class_list>

fix some xml never get origin plain text

before this pr, we need call html.UnescapeString to print the origin plain text
but there is some xml you may never get origin plain text by html.UnescapeString like this xml below

<?xml version="1.0" encoding="utf-8"?>
	<example xml:space="preserve"><word>&amp;#48;		</word></example>

you either get plain text <?xml version=\"1.0\" encoding=\"utf-8\"?><class_list xml:space=\"preserve\">&#xA;&#x9;&#x9;<student>&#xA;&#x9;&#x9;&#x9;<name> Robert </name>&#xA;&#x9;&#x9;&#x9;<grade>A+</grade>&#xA;&#xA;&#x9;&#x9;</student>&#xA;&#x9;</class_list> by OutputXML.

or get <?xml version=\"1.0\" encoding=\"utf-8\"?><example xml:space=\"preserve\"><word>&#48;\t\t</word></example> by OutputXML + html.UnescapeString. None of them are consistent with the original document