fix improper handling of request body backpressure

Closes #348

tests/multipart.rs

@@ -49,3 +49,52 @@ fn test_multipart() {
     assert_eq!(res.url().as_str(), &url);
     assert_eq!(res.status(), reqwest::StatusCode::OK);
 }
+
+#[test]
+fn file() {
+    let _ = env_logger::try_init();
+
+    let form = reqwest::multipart::Form::new()
+        .file("foo", "Cargo.lock").unwrap();
+
+    let fcontents = ::std::fs::read_to_string("Cargo.lock").unwrap();
+
+    let expected_body = format!("\
+        --{0}\r\n\
+        Content-Disposition: form-data; name=\"foo\"; filename=\"Cargo.lock\"\r\n\
+        Content-Type: application/octet-stream\r\n\r\n\
+        {1}\r\n\
+        --{0}--\r\n\
+    ", form.boundary(), fcontents);
+
+    let server = server! {
+        request: format!("\
+            POST /multipart/2 HTTP/1.1\r\n\
+            user-agent: $USERAGENT\r\n\
+            accept: */*\r\n\
+            content-type: multipart/form-data; boundary={}\r\n\
+            content-length: {}\r\n\
+            accept-encoding: gzip\r\n\
+            host: $HOST\r\n\
+            \r\n\
+            {}\
+            ", form.boundary(), expected_body.len(), expected_body),
+        response: b"\
+            HTTP/1.1 200 OK\r\n\
+            Server: multipart\r\n\
+            Content-Length: 0\r\n\
+            \r\n\
+            "
+    };
+
+    let url = format!("http://{}/multipart/2", server.addr());
+
+    let res = reqwest::Client::new()
+        .post(&url)
+        .multipart(form)
+        .send()
+        .unwrap();
+
+    assert_eq!(res.url().as_str(), &url);
+    assert_eq!(res.status(), reqwest::StatusCode::OK);
+}