Change behavior for empty values when deserializing an option

[jplatte]

Currently, the following two forms get the foo field deserialized to None given the input foo=:

#[derive(Deserialize)]
struct FormA {
    foo: Option<String>,
}

#[derive(Deserialize)]
struct FormB {
   foo: Option<Vec<String>>,
}

I think the most reasonable output would instead be an empty string (as the only list element) wrapped in Some for both forms.

What might also be relevant: I think the same forms with String replaced by f64 should produce foo: None for FormA and a deserialization error for FormB. I think both should already be the case.

This is a breaking change, so if implemented I will publish it as part of serde_html_form 0.3.0.

[jplatte]

Should also when working on this consider testing ?foo separately from ?foo=. Both should produce an empty string (see discussion in #21).