Add utility methods for sending non-form data in pkg/http.

pkg/http/test/utils.dart

Index: pkg/http/test/utils.dart
diff --git a/pkg/http/test/utils.dart b/pkg/http/test/utils.dart
index f9a73813ac83756dd26328b0174c8f67c34363e8..f93ca253c159254093f0cc7ce99c5c2f61bd9eaf 100644
--- a/pkg/http/test/utils.dart
+++ b/pkg/http/test/utils.dart
@@ -73,7 +73,8 @@ Future startServer() {
         var requestBody;
         if (requestBodyBytes.isEmpty) {
           requestBody = null;
-        } else if (request.headers.contentType.charset != null) {
+        } else if (request.headers.contentType != null &&
+            request.headers.contentType.charset != null) {
           var encoding = requiredEncodingForCharset(
               request.headers.contentType.charset);
           requestBody = encoding.decode(requestBodyBytes);