Drop undefined/null values from child process env

Undefined LANG was causing an actual LANG="undefined" to be passed
to the child process.

lib/core/map.js

@@ -1,3 +1,5 @@
+var _ = require('underscore');
+
 /*
  * map.js
  *
@@ -17,4 +19,15 @@ exports.create = create;
  */
 function create() {
   return Object.create(null);
+}
+
+exports.compact = compact;
+/**
+ * Return a copy of object x with null or undefined "own" properties removed.
+ */
+function compact(x) {
+  function shouldDrop(key) {
+    return typeof(x[key]) === 'undefined' || x[key] === null;
+  }
+  return _.omit(x, _.filter(_.keys(x), shouldDrop))
 }

lib/worker/app-worker.js

@@ -26,6 +26,7 @@ var util = require('util');
 var bash = require('bash');
 var Q = require('q');
 var _ = require('underscore');
+var map = require('../core/map');
 var paths = require('../core/paths');
 var permissions = require('../core/permissions');
 var posix = require('../../build/Release/posix');
@@ -184,12 +185,12 @@ var AppWorker = function(appSpec, endpoint, logStream, workerId, home) {
       self.$proc = child_process.spawn(executable, args, {
         stdio: ['pipe', 'pipe', logStream],
         cwd: appSpec.appDir,
-        env: {
+        env: map.compact({
           'RSTUDIO_PANDOC': paths.projectFile('ext/pandoc'),
           'HOME' : home,
           'LANG' : process.env['LANG'],
           'PATH' : process.env['PATH']
-        },
+        }),
         detached: true  // So that we can send SIGINT not just to su but to the
                         // R process that it spawns as well
       });