fix(spawn): support string argument

README.md

@@ -498,7 +498,13 @@ Option | Description | Default
 `encoding` | Sets the encoding of the output string | `utf8`
 
 ``` js
-spawn('cat', 'test.txt').then(function(content){
+spawn('cat', 'test.txt').then((content) => {
+  console.log(content);
+});
+
+// $ cd "/target/folder"
+// $ cat "foo.txt" "bar.txt"
+spawn('cat', ['foo.txt', 'bar.txt'], { cwd: '/target/folder' }).then((content) => {
   console.log(content);
 });
 ```

lib/spawn.js

@@ -7,6 +7,8 @@ const CacheStream = require('./cache_stream');
 function promiseSpawn(command, args = [], options) {
   if (!command) throw new TypeError('command is required!');
 
+  if (typeof args === 'string') args = [args];
+
   if (!options && !Array.isArray(args)) {
     options = args;
     args = [];

test/spawn.spec.js

@@ -24,6 +24,8 @@ describe('spawn', () => {
 
   it('default', () => spawn(catCommand, [fixturePath]).should.become(fixture));
 
+  it('default - string', () => spawn(catCommand, fixturePath).should.become(fixture));
+
   it('command is required', () => {
     spawn.should.throw('command is required!');
   });