Regression: params doesn't permit & and # in values

[twm]

The new params merging code in 20.3.0 regressed: it now forbids the characters & and # in param values.

This is because we're using hyperlink.URL (a.k.a. hyperlink.EncodedURL) when we want hyperlink.DecodedURL.

For example:

from twisted.internet.task import react
from twisted.internet.defer import inlineCallbacks

import treq


@inlineCallbacks
def main(reactor):
    print('Preserved query parameters')
    resp = yield treq.get('https://httpbin.org/get?foo=bar',
                          params={'baz': 'b&x'})
    content = yield resp.text()
    print(content)

react(main, [])

Produces an exception:

Traceback (most recent call last):
  File "docs/examples/query_params.py", line 15, in <module>
    react(main, [])
...
  File "docs/examples/query_params.py", line 11, in main
    params={'baz': 'b&x'})
  File "/home/twm/treq/src/treq/api.py", line 24, in get
    return _client(**kwargs).get(url, headers=headers, **kwargs)
  File "/home/twm/treq/src/treq/client.py", line 119, in get
    return self.request('GET', url, **kwargs)
  File "/home/twm/treq/src/treq/client.py", line 172, in request
    query=parsed_url.query + tuple(_coerced_query_params(params))
  File "/home/twm/treq/.tox/py37-twisted_latest/lib/python3.7/site-packages/hyperlink/_url.py", line 1085, in replace
    uses_netloc=_optional(uses_netloc, self.uses_netloc)
  File "/home/twm/treq/.tox/py37-twisted_latest/lib/python3.7/site-packages/hyperlink/_url.py", line 838, in __init__
    for k, v in iter_pairs(query))
  File "/home/twm/treq/.tox/py37-twisted_latest/lib/python3.7/site-packages/hyperlink/_url.py", line 838, in <genexpr>
    for k, v in iter_pairs(query))
  File "/home/twm/treq/.tox/py37-twisted_latest/lib/python3.7/site-packages/hyperlink/_url.py", line 475, in _textcheck
    % (''.join(delims), name, value))
builtins.ValueError: one or more reserved delimiters &# present in query parameter value: 'b&x'

I filed python-hyper/hyperlink#125 about getting the latter documented.