Add tests for xsi:nil handling

failures (11):
    as_field::empty::nested::true_
    as_field::with_attr::nested::true_
    as_field::with_element::nested::true_
    as_field::with_element::ns0::true_
    as_field::with_element::xsi::true_
    top_level_option::empty::ns0::true_
    top_level_option::empty::xsi::true_
    top_level_option::with_attr::ns0::true_
    top_level_option::with_attr::xsi::true_
    top_level_option::with_element::ns0::true_
    top_level_option::with_element::xsi::true_

Author: Mingun

Cargo.toml

@@ -231,6 +231,11 @@ name = "serde-de-seq"
 required-features = ["serialize"]
 path = "tests/serde-de-seq.rs"
 
+[[test]]
+name = "serde-de-xsi"
+required-features = ["serialize"]
+path = "tests/serde-de-xsi.rs"
+
 [[test]]
 name = "serde-se"
 required-features = ["serialize"]