UnicodeDecodeError traceback on URL preview

This issue has been migrated from [#4585](https://github.com/matrix-org/synapse/issues/4585).

---

### Description

UnicodeDecodeError

### Steps to reproduce

```
synapse.http.server - 112 - ERROR - GET-132- Failed handle request via : : Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/usr/local/lib/python3.6/site-packages/synapse/http/server.py", line 81, in wrapped_request_handler
yield h(self, request)
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1613, in unwindGenerator
return _cancellableInlineCallbacks(gen)
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1529, in _cancellableInlineCallbacks
_inlineCallbacks(None, g, status)
--- ---
File "/usr/local/lib/python3.6/site-packages/synapse/http/server.py", line 81, in wrapped_request_handler
yield h(self, request)
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/usr/local/lib/python3.6/site-packages/synapse/rest/media/v1/preview_url_resource.py", line 110, in _async_render_GET
url = parse_string(request, "url")
File "/usr/local/lib/python3.6/site-packages/synapse/http/servlet.py", line 144, in parse_string
request.args, name, default, required, allowed_values, param_type, encoding
File "/usr/local/lib/python3.6/site-packages/synapse/http/servlet.py", line 158, in parse_string_from_args
value = value.decode(encoding)
builtins.UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 86: ordinal not in range(128)
synapse.http.server - 112 - ERROR - GET-134- Failed handle request via : : Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/usr/local/lib/python3.6/site-packages/synapse/http/server.py", line 81, in wrapped_request_handler
yield h(self, request)
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1613, in unwindGenerator
return _cancellableInlineCallbacks(gen)
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1529, in _cancellableInlineCallbacks
_inlineCallbacks(None, g, status)
--- ---
File "/usr/local/lib/python3.6/site-packages/synapse/http/server.py", line 81, in wrapped_request_handler
yield h(self, request)
File "/usr/local/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/usr/local/lib/python3.6/site-packages/synapse/rest/media/v1/preview_url_resource.py", line 110, in _async_render_GET
url = parse_string(request, "url")
File "/usr/local/lib/python3.6/site-packages/synapse/http/servlet.py", line 144, in parse_string
request.args, name, default, required, allowed_values, param_type, encoding
File "/usr/local/lib/python3.6/site-packages/synapse/http/servlet.py", line 158, in parse_string_from_args
value = value.decode(encoding)
builtins.UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 52: ordinal not in range(128)
```

### Version information

- **Version**: 0.99.0
- **Install method**: docker
- **Platform**: Linux