Merge pull request #9 from Wescoeur/prevent-overwrite

createWriteStream supports 'wx'/'w+x' flags.

lib/api/src/createWriteStream.js

@@ -6,7 +6,8 @@ import {Writable} from 'stream'
 import {
   FILE_OPEN,
   FILE_OPEN_IF,
-  FILE_OVERWRITE_IF
+  FILE_OVERWRITE_IF,
+  FILE_CREATE
 } from '../structures/constants'
 
 const requestAsync = Bluebird.promisify(request)
@@ -102,6 +103,8 @@ export default function (path, options, cb) {
     createDisposition = FILE_OPEN_IF
   } else if (flags === 'w' || flags === 'w+') {
     createDisposition = FILE_OVERWRITE_IF
+  } else if (flags === 'wx' || flags === 'w+x') {
+    createDisposition = FILE_CREATE
   }
 
   request('create', { path, createDisposition }, this, (err, file) => {