Implement Default for Client, ClientBuilder, and Form (#811)

src/blocking/client.rs

@@ -63,6 +63,12 @@ pub struct ClientBuilder {
     timeout: Timeout,
 }
 
+impl Default for ClientBuilder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl ClientBuilder {
     /// Constructs a new `ClientBuilder`.
     ///
@@ -455,6 +461,12 @@ impl ClientBuilder {
     }
 }
 
+impl Default for Client {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Client {
     /// Constructs a new `Client`.
     ///