Fixed null signal preservation

Closes GH-674.

src/node.js

@@ -194,13 +194,17 @@
     };
 
     process.kill = function(pid, sig) {
-      sig = sig || 'SIGTERM';
-
-      if (!startup.lazyConstants()[sig]) {
-        throw new Error('Unknown signal: ' + sig);
+      // preserve null signal
+      if (0 === sig) {
+        process._kill(pid, 0);
+      } else {
+        sig = sig || 'SIGTERM';
+        if (startup.lazyConstants()[sig]) {
+          process._kill(pid, startup.lazyConstants()[sig]);
+        } else {
+          throw new Error('Unknown signal: ' + sig);
+        }
       }
-
-      process._kill(pid, startup.lazyConstants()[sig]);
     };
   };
 

test/simple/test-process-kill-null.js

@@ -0,0 +1,20 @@
+
+var assert = require('assert');
+var spawn = require('child_process').spawn;
+
+var cat = spawn('cat');
+var called;
+
+process.kill(cat.pid, 0);
+
+cat.stdout.on('data', function(){
+  called = true;
+  process.kill(cat.pid, 'SIGKILL');
+});
+
+// EPIPE when null sig fails
+cat.stdin.write('test');
+
+process.on('exit', function(){
+  assert.ok(called);
+});