Do not duplicate Last-Modified header

[tonicebrian]

By default when we had metadata, header Last-Modified was being returned twice:

curl -I -X GET localhost:8001/mybucket/hello.txt
HTTP/1.1 200 OK
Accept-Ranges: bytes
Last-Modified: Thu, 01 Jan 1970 00:00:00 GMT
Last-Modified: Thu, 21 Jun 2018 11:44:44 GMT
Timeout-Access: <function1>
Expect: 100-continue
Content-MD5: d41d8cd98f00b204e9800998ecf8427e
Accept-Encoding: identity
User-Agent: aws-cli/1.15.41 Python/3.6.5 Linux/4.15.0-23-generic botocore/1.10.41
Host: localhost:8001
ETag: d41d8cd98f00b204e9800998ecf8427e
Server: akka-http/10.1.0
Date: Thu, 21 Jun 2018 11:45:13 GMT
Content-Type: text/plain
Content-Length: 0

Making some functions from the boto3 library fail with an exception because it is trying to parse as a date the concatenation of those two dates:

In [100]: client.head_object(Key="hello.txt", Bucket="mybucket")                                                                                                                                                   
---------------------------------------------------------------------------                                                                                                                                        
ValueError                                Traceback (most recent call last)                                                                                                                                        
~/.local/share/virtualenvs/hooks_test-2daLLUaP/lib/python3.6/site-packages/botocore/utils.py in parse_timestamp(value)                                                                                             
    371         # enforce that GMT == UTC.                                                                                                                                                                         
--> 372         return dateutil.parser.parse(value, tzinfos={'GMT': tzutc()})                                                                                                                                      
    373     except (TypeError, ValueError) as e:                                                                                                                                                                   

~/.local/share/virtualenvs/hooks_test-2daLLUaP/lib/python3.6/site-packages/dateutil/parser/_parser.py in parse(timestr, parserinfo, **kwargs)
   1355     else:
-> 1356         return DEFAULTPARSER.parse(timestr, **kwargs)
   1357

~/.local/share/virtualenvs/hooks_test-2daLLUaP/lib/python3.6/site-packages/dateutil/parser/_parser.py in parse(self, timestr, default, ignoretz, tzinfos, **kwargs)
    647         if res is None:
--> 648             raise ValueError("Unknown string format:", timestr)
    649

ValueError: ('Unknown string format:', 'Thu, 01 Jan 1970 00:00:00 GMT, Thu, 21 Jun 2018 11:44:44 GMT')

This PR returns a default Last-Modified just when not present.