doctest: Fix regression after upgrade of reqwest to 0.10.

This was introduced in #350. Besides this, my thanks for upgrading
reqwest. The old version cost me some time yesterday.

Author: Christoph Siedentop

README.md

@@ -59,6 +59,7 @@ A typed GraphQL client library for Rust.
   ```rust
   use graphql_client::{GraphQLQuery, Response};
   use std::error::Error;
+  use reqwest;
 
   #[derive(GraphQLQuery)]
   #[graphql(
@@ -68,14 +69,14 @@ A typed GraphQL client library for Rust.
   )]
   pub struct UnionQuery;
 
-  fn perform_my_query(variables: union_query::Variables) -> Result<(), Box<dyn Error>> {
+  async fn perform_my_query(variables: union_query::Variables) -> Result<(), Box<dyn Error>> {
 
       // this is the important line
       let request_body = UnionQuery::build_query(variables);
 
       let client = reqwest::Client::new();
-      let mut res = client.post("/graphql").json(&request_body).send()?;
-      let response_body: Response<union_query::ResponseData> = res.json()?;
+      let mut res = client.post("/graphql").json(&request_body).send().await?;
+      let response_body: Response<union_query::ResponseData> = res.json().await?;
       println!("{:#?}", response_body);
       Ok(())
   }

graphql_client/Cargo.toml

@@ -41,6 +41,7 @@ optional = true
 [dev-dependencies]
 # Note: If we bumpup wasm-bindge-test version, we should change CI setting.
 wasm-bindgen-test = "^0.3"
+reqwest = { version = "^0.10", features = ["json", "blocking"] }
 
 [features]
 default = ["graphql_query_derive"]