XML assignExpression operator does not escape value

[workhorsy]

XML generation is broken with "&=" and "<<=" operators. It does not escape them into "&=" and "<<=". So it generates invalid XML.

./dscanner --ast example.d > example.xml

// example.d
int main() {
size_t a, b;

a &= b; // is:        <assignExpression operator="&=">
        // should be: <assignExpression operator="&amp;=">

a <<= b; // is:        <assignExpression operator="<<=">
         // should be: <assignExpression operator="&lt;&lt;=">

return 0;

}