feat(ethers-core/Chain): implement Default trait (#976)

ethers-core/src/types/chain.rs

@@ -2,7 +2,7 @@ use serde::Deserialize;
 use thiserror::Error;
 
 use core::convert::TryFrom;
-use std::{fmt, str::FromStr};
+use std::{default, fmt, str::FromStr};
 
 use crate::types::U256;
 
@@ -177,3 +177,14 @@ impl Chain {
         )
     }
 }
+
+impl default::Default for Chain {
+    fn default() -> Self {
+        Chain::Mainnet
+    }
+}
+
+#[test]
+fn test_default_chain() {
+    assert_eq!(Chain::default(), Chain::Mainnet);
+}