Escape according to Canonical XML 1.0.

See <http://www.w3.org/TR/2000/WD-xml-c14n-20000119.html#charescaping>.

First and foremost, this change allows adding linefeeds, tabs and carriage returns to attribute values.  This was not possible before, since such characters did not get escaped and without escaping a conforming XML reader will replace them by blanks. [1]

While at it, this change escapes attribute values according to the Canonical XML 1.0 specification.  In particular, in attribute values greater-than no longer gets escaped.

[1] http://www.w3.org/TR/2004/REC-xml-20040204/#AVNormalize