Fix query string escaping in engine URLs

Fix query string escaping in engine URLs

Fixed issue in :class:`.URL` object where stringifying the object
would not URL encode special characters, preventing the URL from being
re-consumable as a real URL.  Pull request courtesy Miguel Grinberg.

Fixes: #5341
Closes: #5342
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5342
Pull-request-sha: 362ca3398336a3a892e8020530f0c68d4f2d1d01

Change-Id: Ief6218122d1ec0c70479eb1a90e1c16433801924
(cherry picked from commit 2644693c0bf5b775f7a7283cdbf566a37872586f)