Fix response of PUT Object Copy requests

Current Swift3 does not return response body.
Also, response body that should be returned and headers are incorrect.

So, I added the following fixes.
 - fix Swift3 to retrun response body
 - add LastModified element to response body
 - change the value of Content-Type in response headers to application/xml
 - delete Etag from response headers

Also, this patch is related to the following patch.
 - https://review.openstack.org/#/c/126403

Change-Id: I41f6a4ed2445a1473cd766b0fe60d6894288b7dd

swift3/controllers/obj.py

@@ -17,8 +17,7 @@ from swift.common.http import HTTP_OK
 from swift.common.swob import Range, content_range_header_value
 
 from swift3.controllers.base import Controller
-from swift3.response import HTTPOk, S3NotImplemented, InvalidRange,\
-    HTTPPartialContent
+from swift3.response import S3NotImplemented, InvalidRange, HTTPPartialContent
 from swift3.etree import Element, SubElement, tostring
 
 
@@ -99,9 +98,12 @@ class ObjectController(Controller):
 
         if 'X-Amz-Copy-Source' in req.headers:
             elem = Element('CopyObjectResult')
+            SubElement(elem, 'LastModified').text = \
+                resp.last_modified.isoformat()[:-6] + '.000Z'
             SubElement(elem, 'ETag').text = '"%s"' % resp.etag
-            body = tostring(elem, use_s3ns=False)
-            return HTTPOk(body=body, headers=resp.headers)
+            resp.body = tostring(elem)
+            resp.headers['Content-Type'] = 'application/xml'
+            resp.etag = None
 
         resp.status = HTTP_OK
         return resp