Add support for #[serde(default)] on structs

This allows structs to use the default value for each field defined in
the struct’s `std::default::Default` implementation, rather then the
default value for the field’s type.

```
struct StructDefault {
    a: i32,
    b: String,
}

impl Default for StructDefault {
    fn default() -> StructDefault {
        StructDefault{
            a: 100,
            b: "default".to_string(),
        }
    }
}
```

The code above will now return `100` for field `a` and `”default”` for
`b`, rather then `0` and `””` respectively.

test_suite/tests/test_de.rs

@@ -2,6 +2,7 @@ use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::net;
 use std::path::PathBuf;
 use std::time::Duration;
+use std::default::Default;
 
 use serde::Deserialize;
 
@@ -40,6 +41,22 @@ struct StructDenyUnknown {
     b: i32,
 }
 
+#[derive(PartialEq, Debug, Deserialize)]
+#[serde(default)]
+struct StructDefault {
+    a: i32,
+    b: String,
+}
+
+impl Default for StructDefault {
+    fn default() -> StructDefault {
+        StructDefault{
+            a: 100,
+            b: "default".to_string(),
+        }
+    }
+}
+
 #[derive(PartialEq, Debug, Deserialize)]
 struct StructSkipAll {
     #[serde(skip_deserializing)]
@@ -728,6 +745,23 @@ declare_tests! {
             Token::StructEnd,
         ],
     }
+    test_struct_default {
+        StructDefault{ a: 50, b: "overwritten".to_string() } => &[
+            Token::StructStart("StructDefault", 1),
+                Token::StructSep,
+                Token::Str("a"),
+                Token::I32(50),
+
+                Token::StructSep,
+                Token::Str("b"),
+                Token::String("overwritten".to_string()),
+            Token::StructEnd,
+        ],
+        StructDefault{ a: 100, b: "default".to_string() } => &[
+            Token::StructStart("StructDefault", 0),
+            Token::StructEnd,
+        ],
+    }
     test_enum_unit {
         Enum::Unit => &[
             Token::EnumUnit("Enum", "Unit"),