Add https_only() for ClientBuilder (#1102)

Closes #980

src/async_impl/client.rs

@@ -106,6 +106,7 @@ struct Config {
     cookie_store: Option<cookie::CookieStore>,
     trust_dns: bool,
     error: Option<crate::Error>,
+    https_only: bool,
 }
 
 impl Default for ClientBuilder {
@@ -157,6 +158,7 @@ impl ClientBuilder {
                 trust_dns: cfg!(feature = "trust-dns"),
                 #[cfg(feature = "cookies")]
                 cookie_store: None,
+                https_only: false,
             },
         }
     }
@@ -349,6 +351,7 @@ impl ClientBuilder {
                 request_timeout: config.timeout,
                 proxies,
                 proxies_maybe_http_auth,
+                https_only: config.https_only,
             }),
         })
     }
@@ -917,6 +920,14 @@ impl ClientBuilder {
             self
         }
     }
+
+    /// Restrict the Client to be used with HTTPS only requests.
+    /// 
+    /// Defaults to false.
+    pub fn https_only(mut self, enabled: bool) -> ClientBuilder {
+        self.config.https_only = enabled;
+        self
+    }
 }
 
 type HyperClient = hyper::Client<Connector, super::body::ImplStream>;
@@ -1040,6 +1051,11 @@ impl Client {
             return Pending::new_err(error::url_bad_scheme(url));
         }
 
+        // check if we're in https_only mode and check the scheme of the current URL
+        if self.inner.https_only && url.scheme() != "https" {
+            return Pending::new_err(error::url_bad_scheme(url));
+        }
+
         // insert default headers in the request headers
         // without overwriting already appended headers.
         for (key, value) in &self.inner.headers {
@@ -1238,6 +1254,7 @@ struct ClientRef {
     request_timeout: Option<Duration>,
     proxies: Arc<Vec<Proxy>>,
     proxies_maybe_http_auth: bool,
+    https_only: bool,
 }
 
 impl ClientRef {