Fix improper comparison of numeric default attribute values with textvalues resulting in wrongly removed attributes (#101)

For example for `orient="auto"` SVGLength() returns (value=0, units=Unit.INVALID); since the default value for `orient` is zero it was removed as there was check for a valid unit.

unittests/orient-marker.svg

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+ <defs>
+  <marker id="m1" orient="auto">
+   <rect width="200" height="100"/>
+  </marker>
+  <marker id="m2" orient="0">
+   <rect width="200" height="100"/>
+  </marker>
+ </defs>
+ <line x2="100" y2="100" style="marker-start:url(#m1);marker-end:url(#m2)" stroke="#000" />
+</svg>