Incorrect UTF-8 with high plane characters

[matthiasr]

When trying to save a dashboard with a 𝚫 I got a 500 from the server. Server log says:

15688556    2015-07-03_16:21:49.50274 I, [2015-07-03T16:21:43.719763 #5]  INFO -- : Completed 500 Internal Server Error in 7ms
15688556    2015-07-03_16:21:49.50274 F, [2015-07-03T16:21:43.722726 #5] FATAL -- :
15688556    2015-07-03_16:21:49.50275 JSON::GeneratorError (source sequence is illegal/malformed utf-8):
15688556    2015-07-03_16:21:49.50275   app/controllers/dashboards_controller.rb:128:in `dashboard_params'
15688556    2015-07-03_16:21:49.50275   app/controllers/dashboards_controller.rb:85:in `block in update'
15688556    2015-07-03_16:21:49.50276   app/controllers/dashboards_controller.rb:84:in `update'
15688556    2015-07-03_16:21:49.50276
15688556    2015-07-03_16:21:49.50277

The string in question (legend string) was encoded as "\ud835\udeab" in the JSON sent by the browser.

[matthiasr]

Sending a Δ instead works.