Make the async Client default (#626)

The previously default Client is moved to `reqwest::blocking`, while the
async client becomes the main API.

Closes #622

tests/badssl.rs

@@ -1,57 +1,65 @@
 #[cfg(feature = "tls")]
-#[test]
-fn test_badssl_modern() {
+#[tokio::test]
+async fn test_badssl_modern() {
     let text = reqwest::get("https://mozilla-modern.badssl.com/")
+        .await
         .unwrap()
         .text()
+        .await
         .unwrap();
 
     assert!(text.contains("<title>mozilla-modern.badssl.com</title>"));
 }
 
 #[cfg(feature = "rustls-tls")]
-#[test]
-fn test_rustls_badssl_modern() {
+#[tokio::test]
+async fn test_rustls_badssl_modern() {
     let text = reqwest::Client::builder()
         .use_rustls_tls()
         .build()
         .unwrap()
         .get("https://mozilla-modern.badssl.com/")
         .send()
+        .await
         .unwrap()
         .text()
+        .await
         .unwrap();
 
     assert!(text.contains("<title>mozilla-modern.badssl.com</title>"));
 }
 
 #[cfg(feature = "tls")]
-#[test]
-fn test_badssl_self_signed() {
+#[tokio::test]
+async fn test_badssl_self_signed() {
     let text = reqwest::Client::builder()
         .danger_accept_invalid_certs(true)
         .build()
         .unwrap()
         .get("https://self-signed.badssl.com/")
         .send()
+        .await
         .unwrap()
         .text()
+        .await
         .unwrap();
 
     assert!(text.contains("<title>self-signed.badssl.com</title>"));
 }
 
 #[cfg(feature = "default-tls")]
-#[test]
-fn test_badssl_wrong_host() {
+#[tokio::test]
+async fn test_badssl_wrong_host() {
     let text = reqwest::Client::builder()
         .danger_accept_invalid_hostnames(true)
         .build()
         .unwrap()
         .get("https://wrong.host.badssl.com/")
         .send()
+        .await
         .unwrap()
         .text()
+        .await
         .unwrap();
 
     assert!(text.contains("<title>wrong.host.badssl.com</title>"));
@@ -61,7 +69,8 @@ fn test_badssl_wrong_host() {
         .build()
         .unwrap()
         .get("https://self-signed.badssl.com/")
-        .send();
+        .send()
+        .await;
 
     assert!(result.is_err());
 }