`with_query` autoencode unexpected result

[dimaqq]

I've ran into this when some external service sent an escaped character:

In [16]: str(URL("/").with_query(foo="bar&baz%26qux"))
Out[16]: '/?foo=bar%26baz%26qux'

The distinction between inputs & and %26 is lost:

In [19]: URL(x).query
Out[19]: <MultiDictProxy('foo': 'bar&baz&qux')>

This makes the library hard to use for arbitrary input.

The specific case where yarl broke involved 2 redirects (i.e. nested) where each had own query parameters.