fix(build): fix with_interceptor not building on Rust 1.51

Fixes #666

Author: davidpdrsn

tonic-build/src/client.rs

@@ -50,7 +50,10 @@ pub fn generate<T: Service>(
                 pub fn with_interceptor<F>(inner: T, interceptor: F) -> #service_ident<InterceptedService<T, F>>
                 where
                     F: FnMut(tonic::Request<()>) -> Result<tonic::Request<()>, tonic::Status>,
-                    T: Service<http::Request<tonic::body::BoxBody>, Response = http::Response<T::ResponseBody>>,
+                    T: Service<
+                        http::Request<tonic::body::BoxBody>,
+                        Response = http::Response<<T as tonic::client::GrpcService<tonic::body::BoxBody>>::ResponseBody>
+                    >,
                     <T as Service<http::Request<tonic::body::BoxBody>>>::Error: Into<StdError> + Send + Sync,
                 {
                     #service_ident::new(InterceptedService::new(inner, interceptor))