Fixing multi value headers so they are serialized correctly

dio/lib/src/adapters/io_adapter.dart

@@ -56,7 +56,7 @@ class IOHttpClientAdapter implements HttpClientAdapter {
 
       //Set Headers
       options.headers.forEach((k, v) {
-        if (v != null) request.headers.set(k, '$v');
+        if (v != null) request.headers.set(k, v);
       });
     } on SocketException catch (e, stackTrace) {
       if (!e.message.contains('timed out')) {

dio/test/request_test.dart

@@ -86,6 +86,22 @@ void main() {
       assert(ri.method == 'GET');
     });
 
+    test('#test multi value headers', () async {
+      Response response = await dio.get(
+        '/multi-value-header',
+        options: Options(
+          headers: {
+            'x-multi-value-request-header': ['value1', 'value2'],
+          },
+        ),
+      );
+      expect(response.statusCode, 200);
+      expect(
+        response.headers.value('x-multi-value-request-header-echo'),
+        equals('value1, value2'),
+      );
+    });
+
     test('#test request with URI', () async {
       Response response;
 

dio/test/utils.dart

@@ -74,6 +74,20 @@ Future<void> startServer() async {
         return;
       }
 
+      if (path == '/multi-value-header') {
+        response.headers.contentType = ContentType('application', 'json');
+        response.headers.add(
+          'x-multi-value-request-header-echo',
+          request.headers.value('x-multi-value-request-header').toString(),
+        );
+        response
+          ..statusCode = 200
+          ..contentLength = -1
+          ..write('');
+        response.close();
+        return;
+      }
+
       if (path == '/download') {
         const content = 'I am a text file';
         response.headers.set('content-encoding', 'plain');