UrlDispatcher error during constructing of url with trailing slash

It makes urls without trailing slash.It happened after fixing of regex error :)

```
>>> from aiohttp.web import UrlDispatcher
>>> disp = UrlDispatcher()
>>> disp.add_route('GET', '/hello/{name}/', lambda x: x, name='name')
<DynamicRoute 'name' [GET] /hello/{name} -> <function <lambda> at 0x7ffd8b47b840>
>>> disp['name'].url(parts={'name': 'foo'})
'/hello/foo'
```
