This change modifies the table entries for ctx_special_attr to treat TAB
character as special, which makes the output code escape it.

Before this change, trying to use TAB in an attribute value would output
it verbatim; during subsequent parsing, pugixml - and other compliant
parsers - would apply attribute-value normalization, turning the TAB
into a space and losing the original value.

Using 	 fixes this; if an input document has 	 in an attribute
value, that gets unescaped into \t during parsing and escaped back into
	 during output, which means we can now roundtrip values like this.

Fixes #242.