Just lstrip instead of strip because trailing whitespace already works.

requests/models.py

@@ -343,8 +343,8 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         else:
             url = unicode(url) if is_py2 else str(url)
 
-        # Remove leading and trailing whitespaces from url
-        url = url.strip()
+        # Remove leading whitespaces from url
+        url = url.lstrip()
 
         # Don't do any URL preparation for non-HTTP schemes like `mailto`,
         # `data` etc to work around exceptions from `url_parse`, which

tests/test_requests.py

@@ -142,7 +142,7 @@ class TestRequests:
 
     def test_whitespaces_are_removed_from_url(self):
         # Test for issue #3696
-        request = requests.Request('GET', ' http://example.com ').prepare()
+        request = requests.Request('GET', ' http://example.com').prepare()
         assert request.url == 'http://example.com/'
 
     @pytest.mark.parametrize('scheme', ('http://', 'HTTP://', 'hTTp://', 'HttP://'))