Add support for Option<struct> in 'attribute' fields

[DmitrySamoylov]

Hey @MarcAntoine-Arnaud !

Now it is possible to use Option<struct> or Option<enum> in fields marked as attribute:

  #[derive(YaDeserialize, YaSerialize, PartialEq, Debug)]
  enum AttrEnum {
    Variant1,
    Variant2,
  }

  #[derive(YaDeserialize, YaSerialize, Default, PartialEq, Debug)]
  struct Struct {
    #[yaserde(attribute)]
    attr_option_enum: Option<AttrEnum>,
  }

Corresponding XML can be like:

<?xml version="1.0" encoding="utf-8"?>
<Struct attr_option_enum="Variant2" />

[coveralls]

Pull Request Test Coverage Report for Build 117

• 12 of 64 (18.75%) changed or added relevant lines in 5 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 58.065%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
yaserde/tests/deserializer.rs	6	8	75.0%
yaserde/tests/serializer.rs	6	9	66.67%
yaserde_derive/src/ser/expand_struct.rs	0	4	0.0%
yaserde_derive/src/field_type.rs	0	5	0.0%
yaserde_derive/src/de/expand_struct.rs	0	38	0.0%

Files with Coverage Reduction	New Missed Lines	%
yaserde_derive/src/de/expand_struct.rs	1	0%
yaserde_derive/src/ser/expand_struct.rs	1	0%

Totals
Change from base Build 116:	0.2%
Covered Lines:	864
Relevant Lines:	1488

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💛 - Coveralls