Merge pull request #1 from firesh/master

修复当option.prefix=''的时候，报SignatureDoesNotMatch错误的问题

Author: jqnote

README.md

@@ -15,7 +15,7 @@ gulp.task('oss', function(){
         secretAccessKey: '*********',
         endpoint: 'http://oss-cn-***.aliyuncs.com',
         apiVersion: '2013-10-15',
-        prefix: 'assets/js',
+        prefix: 'assets/js', //for no prefix: prefix: ''
         bucket: 'test'
     };
     //./js/a.js -> <prefix>/a.js

index.js

@@ -39,7 +39,7 @@ function oss(option) {
         }
         var getFileKey = function(){
             return option.prefix
-                + ((!option.prefix || option.prefix[option.prefix.length - 1]) === '/' ? '' : '/')
+                + ((!option.prefix || (option.prefix[option.prefix.length - 1]) === '/') ? '' : '/')
                 + (option.versioning ? version + '/' : '')
                 + path.relative(file.base, file.path);
         };