test: add test for default type pamameters (#300)

Co-authored-by: Ch.-David Blot <charles-david.blot@orange.com>

validator_derive_tests/tests/run-pass/default_struct_parameters.rs

@@ -0,0 +1,8 @@
+use validator::Validate;
+
+#[derive(Validate)]
+struct Test<T = ()> {
+    a: T,
+}
+
+fn main() {}