feat(writeFile): support flags

`flag` option is also supported for `fs` compatibility

lib/api/writeFile.js

 var SMB2Forge = require('../tools/smb2-forge');
 var SMB2Request = SMB2Forge.request;
 var BigInt = require('../tools/bigint');
+var constants = require('../structures/constants');
 
 /*
  * writeFile
@@ -23,6 +24,16 @@ module.exports = function writeFile(filename, data, options, cb) {
     options = {};
   }
 
+  var createDisposition;
+  var flags = options != null && (options.flags || options.flag);
+  if (flags === 'r+') {
+    createDisposition = constants.FILE_OPEN;
+  } else if (flags === 'w' || flags === 'w+') {
+    createDisposition = constants.FILE_OVERWRITE_IF;
+  } else if (flags === 'wx' || flags === 'wx+') {
+    createDisposition = constants.FILE_CREATE;
+  }
+
   options.encoding = options.encoding || 'utf8';
 
   var connection = this;
@@ -33,14 +44,19 @@ module.exports = function writeFile(filename, data, options, cb) {
   var fileLength = new BigInt(8, fileContent.length);
 
   function createFile(fileCreated) {
-    SMB2Request('create', { path: filename }, connection, function(err, f) {
-      if (err) cb && cb(err);
-      // SMB2 set file size
-      else {
-        file = f;
-        fileCreated();
+    SMB2Request(
+      'create',
+      { createDisposition: createDisposition, path: filename },
+      connection,
+      function(err, f) {
+        if (err) cb && cb(err);
+        // SMB2 set file size
+        else {
+          file = f;
+          fileCreated();
+        }
       }
-    });
+    );
   }
 
   function closeFile(fileClosed) {