Fix broken testErrorResponse unit test

Author: Peter Nied

onedrivesdk/src/androidTest/java/com/onedrive/sdk/http/DefaultHttpProviderTests.java

@@ -306,7 +306,9 @@ public String getJsonResponse() {
 
             @Override
             public Map<String, String> getHeaders() {
-                return new HashMap<>();
+                final HashMap<String, String> headers = new HashMap<>();
+                headers.put("Content-Type", "application/json");
+                return headers;
             }
         };
         mProvider.setConnectionFactory(new MockSingleConnectionFactory(new TestDataConnection(data)));

onedrivesdk/src/main/java/com/onedrive/sdk/http/OneDriveServiceException.java

@@ -300,7 +300,8 @@ public static <T> OneDriveServiceException createFromConnection(final IHttpReque
         OneDriveErrorResponse error = null;
         Exception parsingException = null;
 
-        if (headers.get(DefaultHttpProvider.ContentTypeHeaderName).contains(DefaultHttpProvider.JsonContentType)) {
+        final String contentType = headers.get(DefaultHttpProvider.ContentTypeHeaderName);
+        if (contentType != null && contentType.contains(DefaultHttpProvider.JsonContentType)) {
             try {
                 error = serializer.deserializeObject(rawOutput, OneDriveErrorResponse.class);
             } catch (final Exception ex) {