Fix date format in copy conditions (minio#623)

Fix for x-amz-copy-source-if-modified-since and x-amz-copy-source-if-
unmodified-since values being sent in the wrong format to the server.
The server expects the date format to be in http.TimeFormat.
Also Fixed the format of the sample date in the unit test.

Fixes minio#620

src/main/minio.js

@@ -2053,14 +2053,14 @@ export class CopyConditions {
     if (!(date instanceof Date))
       throw new TypeError('date must be of type Date')
 
-    this.modified = date.toISOString()
+    this.modified = date.toUTCString()
   }
 
   setUnmodified(date) {
     if (!(date instanceof Date))
       throw new TypeError('date must be of type Date')
 
-    this.unmodified = date.toISOString()
+    this.unmodified = date.toUTCString()
   }
 
   setMatchETag(etag) {

src/test/unit/test.js

@@ -60,7 +60,7 @@ describe('Helpers', () => {
 })
 
 describe('CopyConditions', () => {
-  let date = '2017-08-11T19:34:18.437Z'
+  let date = 'Fri, 11 Aug 2017 19:34:18 GMT'
 
   let cc = new Minio.CopyConditions()