fix(form): Set default value for form fields (#4047)

- Use specified default value in struct tags when binding a request input to struct for validation, even if sent empty, not only when not sent at all.
- Add string field to `TestMappingDefault` test case.
- Add test case for not sent form field to default to the value specified via code.
- Add test case for form field sent empty to default to the value specified via code.

Fixes: How to apply default value if empty value provided by client during model binding? #4042, #13042df, #a41721a

binding/form_mapping.go

@@ -261,6 +261,9 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
 
 		if len(vs) > 0 {
 			val = vs[0]
+			if val == "" {
+				val = opt.defaultValue
+			}
 		}
 		if ok, err := trySetCustom(val, value); ok {
 			return ok, err