Test cases fail when HTTPBIN_URL does not have trailing slash

Test cases can be run against a local httpbin server defined by
the `HTTPBIN_URL` environment variable, but it causes tests to
fail if the given URL does not end with a slash.

Ensure that the URL always ends with a slash.

test_requests.py

@@ -23,6 +23,8 @@ except ImportError:
     import io as StringIO
 
 HTTPBIN = os.environ.get('HTTPBIN_URL', 'http://httpbin.org/')
+# Issue #1483: Make sure the URL always has a trailing slash
+HTTPBIN = HTTPBIN.rstrip('/') + '/'
 
 
 def httpbin(*suffix):