fix: package.main with -- arguments

Fixes #1758

The combination of using a package.main (which sets the script
position to index zero) and using the -- stop slurp meant that
the arguments had the script appended to the end instead of
prepended to the start. The net result meant that when the script
was forked, it would drop the first user arg.

See diff for details of the fix - a simple check against null.

lib/config/load.js

@@ -74,7 +74,9 @@ function load(settings, options, config, callback) {
           }
           // if the script is found as a result of not being on the command
           // line, then we move any of the pre double-dash args in execArgs
-          const n = options.scriptPosition || options.args.length;
+          const n = options.scriptPosition === null ?
+            options.args.length : options.scriptPosition;
+
           options.execArgs = (options.execArgs || [])
             .concat(options.args.splice(0, n));
           options.scriptPosition = null;

lib/monitor/run.js

@@ -97,6 +97,8 @@ function run(options) {
     utils.version.major > 4 // only fork if node version > 4
 
   if (shouldFork) {
+    // this assumes the first argument is the script and slices it out, since
+    // we're forking
     var forkArgs = cmd.args.slice(1);
     var env = utils.merge(options.execOptions.env, process.env);
     stdio.push('ipc');

test/config/load.test.js

@@ -298,4 +298,27 @@ describe('config load', function () {
       done();
     })
   });
+
+  it('should support pkg.main and keep user args on args', done => {
+    process.chdir(path.resolve(pwd, 'test/fixtures/packages/main-and-start'));
+    const settings = { scriptPosition: 0, script: null, args: [ 'first', 'second' ] };
+    const options = { ignore: [], watch: [], monitor: [] };
+    const config = {
+      run: false,
+      system: { cwd: '/Users/remy/dev/nodemon/issues/1758' },
+      required: false,
+      dirs: [],
+      timeout: 1000,
+      options: { ignore: [], watch: [], monitor: [] },
+      lastStarted: 0,
+      loaded: []
+    }
+
+    load(settings, options, config, res => {
+      assert.deepEqual(res.execOptions.args, ['first', 'second']);
+      done();
+    })
+  });
+
+
 });

test/fixtures/packages/main-and-start/package.json

@@ -0,0 +1,6 @@
+{
+  "main": "./index.js",
+  "scripts": {
+    "start": "node index.js"
+  }
+}