Adding the functionality and test so that the If-Modified-Since header is honored in GET Object

Signed-off-by: Aden Khan <aakhan@sep.com>

tests/test_s3/test_s3.py

@@ -1596,6 +1596,28 @@ def test_boto3_delete_versioned_bucket():
 
     client.delete_bucket(Bucket='blah')
 
+@mock_s3
+def test_boto3_get_object_if_modified_since():
+    s3 = boto3.client('s3', region_name='us-east-1')
+    bucket_name = "blah"
+    s3.create_bucket(Bucket=bucket_name)
+
+    key = 'hello.txt'
+
+    s3.put_object(
+        Bucket=bucket_name,
+        Key=key,
+        Body='test'
+    )
+
+    with assert_raises(botocore.exceptions.ClientError) as err:
+        s3.get_object(
+            Bucket=bucket_name,
+            Key=key,
+            IfModifiedSince=datetime.datetime.utcnow() + datetime.timedelta(hours=1)
+        )
+    e = err.exception
+    e.response['Error'].should.equal({'Code': '304', 'Message': 'Not Modified'})
 
 @mock_s3
 def test_boto3_head_object_if_modified_since():