Adds null check for status before checking status code. Avoids throwing exceptions if status is not defined. (#145)

* Adds check for status in responseMap as part of error checking.

* formats file

lib/io.dart

diff --git a/lib/io.dart b/lib/io.dart
index ba37a29..e06b1ba 100644
--- a/lib/io.dart
+++ b/lib/io.dart
@@ -103,7 +103,9 @@
 
     if (response.statusCode < 200 ||
         response.statusCode > 299 ||
-        (respBody is Map && respBody['status'] != 0)) {
+        (respBody is Map 
+          && respBody['status'] != null 
+          && respBody['status'] != 0)) {
       throw new WebDriverException(
           httpStatusCode: response.statusCode,
           httpReasonPhrase: response.reasonPhrase,