Fix using incorrect deserializer when deserialize Option values in maps and structs

[Mingun]

Fixes #510. The reason for the error is that an incorrect deserializer was used after forwarding. The fix changes all methods that can forward actual deserialization to another deserializer, like that:

    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, DeError>
    where
        V: Visitor<'de>,
    {
        match self.peek()? {
            DeEvent::Text(t) if t.is_empty() => visitor.visit_none(),
            DeEvent::CData(t) if t.is_empty() => visitor.visit_none(),
            DeEvent::Eof => visitor.visit_none(),
            _ => visitor.visit_some(self), // <<< Forwarding deserialization to another deserializer
        }
    }

[codecov-commenter]

Codecov Report

Merging #548 (06d7207) into master (add7406) will increase coverage by 0.07%.
The diff coverage is 95.91%.

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@@            Coverage Diff             @@
##           master     #548      +/-   ##
==========================================
+ Coverage   61.28%   61.35%   +0.07%     
==========================================
  Files          32       32              
  Lines       15639    15605      -34     
==========================================
- Hits         9584     9575       -9     
+ Misses       6055     6030      -25     

Flag	Coverage Δ
unittests	61.35% <95.91%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/de/map.rs	80.58% <50.00%> (+7.77%)	⬆️
src/de/mod.rs	56.20% <100.00%> (-0.66%)	⬇️
src/reader/mod.rs	94.94% <0.00%> (-0.11%)	⬇️
src/events/attributes.rs	93.56% <0.00%> (+0.06%)	⬆️
src/escapei.rs	12.58% <0.00%> (+0.17%)	⬆️
src/lib.rs	13.52% <0.00%> (+0.23%)	⬆️

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