add content type header for the open in editor toolbar button (#229)

Lack of content header was causing Error 400 bad request errors for the open in editor

Author: emily8rown

front_end/panels/sources/UISourceCodeFrame.ts

@@ -106,6 +106,9 @@ export class UISourceCodeFrame extends
 
         fetch('/open-stack-frame', {
           method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+          },
           body: JSON.stringify(body),
         }).catch(e => console.error(e));
       });