Implement boolean simplification suggested by @iconexperiene

See https://github.com/paperjs/paper.js/issues/1054#issuecomment-222517963

src/path/PathItem.Boolean.js

@@ -35,7 +35,7 @@ PathItem.inject(new function() {
     // contribution contributes to the final result or not. They are applied to
     // for each segment after the paths are split at crossings.
     var operators = {
-        unite:     { 0: true, 1: true },
+        unite:     { 1: true },
         intersect: { 2: true },
         subtract:  { 1: true },
         exclude:   { 1: true }