Throw more specific exception for a request on a close IOClient (#584)

Closes #581

Author: guyluz11

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## 0.13.4-dev
 
+* Throw a more useful error when a client is used after it has been closed.
+
 ## 0.13.3
 
 * Validate that the `method` parameter of BaseRequest is a valid "token".

lib/src/io_client.dart

@@ -24,6 +24,11 @@ class IOClient extends BaseClient {
   /// Sends an HTTP request and asynchronously returns the response.
   @override
   Future<IOStreamedResponse> send(BaseRequest request) async {
+    if (_inner == null) {
+      throw ClientException(
+          'HTTP request failed. Client is already closed.', request.url);
+    }
+
     var stream = request.finalize();
 
     try {