[lib] Change boto2 to not override the User Agent if already present

This enables to reuse the boto HttpRequest client with Raz signed URLs.
The Raz URLs are signed and the User Agent is part of the signature,
so this one should not be changed.

Author: romainr

desktop/libs/aws/src/aws/s3/s3connection_test.py

@@ -52,9 +52,11 @@ def test_list_buckets(self):
     with patch('aws.s3.s3connection.S3RazClient.get_url') as get_url:
       with patch('aws.s3.s3connection.RazS3Connection._mexe') as _mexe:
 
-        get_url.return_value = 'https://gethue-test.s3.amazonaws.com/?' + \
-            'AWSAccessKeyId=AKIA23E77ZX2HVY76YGL' + \
-            '&Signature=3lhK%2BwtQ9Q2u5VDIqb4MEpoY3X4%3D&Expires=1617207304'
+        get_url.return_value = {
+            'AWSAccessKeyId': 'AKIA23E77ZX2HVY76YGL',
+            'Signature': '3lhK%2BwtQ9Q2u5VDIqb4MEpoY3X4%3D',
+            'Expires': '1617207304'
+        }
         _mexe.return_value = ['<Bucket: demo-gethue>', '<Bucket: gethue-test>']
 
         client = RazS3Connection(host='s3-us-west-1.amazonaws.com')
@@ -73,12 +75,15 @@ def test_list_buckets(self):
           's3-us-west-1.amazonaws.com:443' if sys.version_info[0] > 2 else 's3-us-west-1.amazonaws.com',
           http_request.host
         )
-        assert_equal(
-          '/?AWSAccessKeyId=AKIA23E77ZX2HVY76YGL&Signature=3lhK%2BwtQ9Q2u5VDIqb4MEpoY3X4%3D&Expires=1617207304',
-          http_request.path
-        )
+        assert_equal('/', http_request.path)
         assert_equal('/', http_request.auth_path)
-        assert_equal({}, http_request.headers )
+        assert_equal({
+            'AWSAccessKeyId': 'AKIA23E77ZX2HVY76YGL',
+            'Signature': '3lhK%2BwtQ9Q2u5VDIqb4MEpoY3X4%3D',
+            'Expires': '1617207304'
+          },
+          http_request.headers
+        )
         assert_equal({}, http_request.params)
         assert_equal('', http_request.body)