Rename option to “pool_max_idle_per_host” (#917)

reqwest exposes the “pool_max_idle_per_host” option of hyper’s client
builder. This option used to be called “max_idle_per_host” in the hyper
crate, but it has recently been renamed [1].

This patch renames the reqwest representation of this option to make it
consistent with its name in the hyper crate again.

[1] https://github.com/hyperium/hyper/pull/2142

src/blocking/client.rs

@@ -309,8 +309,14 @@ impl ClientBuilder {
     }
 
     /// Sets the maximum idle connection per host allowed in the pool.
+    pub fn pool_max_idle_per_host(self, max: usize) -> ClientBuilder {
+        self.with_inner(move |inner| inner.pool_max_idle_per_host(max))
+    }
+
+    #[doc(hidden)]
+    #[deprecated(note = "use pool_max_idle_per_host instead")]
     pub fn max_idle_per_host(self, max: usize) -> ClientBuilder {
-        self.with_inner(move |inner| inner.max_idle_per_host(max))
+        self.pool_max_idle_per_host(max)
     }
 
     /// Enable case sensitive headers.