Fix duplication when adding the same query key multiple times

[sleeyax]

Currently, the EncodeWithOrder method duplicates the resulting query parameters when the same key was added multiple times:

values := Values{}
values.Add("xyz", "abc")
values.Add("abc", "xyz")
values.Add("foo", "bar")
values.Add("foo", "baa")
values.Add("foo", "baz") 
encoded := values.EncodeWithOrder()
// encoded:
// xyz=abc&abc=xyz&foo=bar&foo=baa&foo=baz&foo=bar&foo=baa&foo=baz&foo=bar&foo=baa&foo=baz
// 
// the expected value is:
// xyz=abc&abc=xyz&foo=bar&foo=baa&foo=baz

This PR resolves the issue.

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See changed tests for details.