Ignore color in invalid numeric format

- fixes #342

Source/Painting/SvgColourConverter.cs

@@ -155,7 +155,13 @@ namespace Svg
                         case "windowframe": return SystemColors.WindowFrame;
                         case "windowtext": return SystemColors.WindowText;
                     }
-
+                    int number;
+                    if (Int32.TryParse(colour, out number))
+                    {
+                        // numbers are handled as colors by System.Drawing.ColorConverter - we
+                        // have to prevent this and ignore the color instead (see #342) 
+                        return SvgColourServer.NotSet;
+                    }
                 }
                 finally
                 {

Tests/Svg.UnitTests/PassingTests.csv

@@ -186,6 +186,7 @@ __issue-323-02
 __issue-323-03
 __issue-329-01
 __issue-338-01_stroke_width
+__issue-342-01
 __issue-354-01
 __issue-385-01_Test_text-anchor-middle
 __issue-398-01

Tests/W3CTestSuite/svg/__issue-342-01.svg

 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
-<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="45" height="45">
-  <g style="opacity:1; fill:000000; fill-opacity:1; fill-rule:evenodd; stroke:#000000; stroke-width:1.5; stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4; stroke-dasharray:none; stroke-opacity:1;">
+<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="180" height="180">
+  <g style="opacity:1; fill:000000; fill-opacity:1; fill-rule:evenodd; stroke:#000000; stroke-width:1.5; stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4; stroke-dasharray:none; stroke-opacity:1;" transform="scale(4)">
     <g style="fill:#000000; stroke:none;">
       <circle cx="6"    cy="12" r="2.75" />
       <circle cx="14"   cy="9"  r="2.75" />