Avoid a django breaking change: set_language response code

In Django 1.10+, the set_language view (/i18n/setlang) will respond to
Ajax requests which do not contain a "next" parameter with status 204 No
Content instead of 304.  This commit adds the "next" parameter to the
request in order to ensure that upgrading to Django 1.10+ will not cause
the set_language view to change behavior.

PLAT-1353

lms/static/js/spec/student_account/account_settings_fields_spec.js

@@ -142,8 +142,13 @@ define(['backbone',
                     requests,
                     'POST',
                     '/i18n/setlang/',
-                    'language=' + data[fieldData.valueAttribute]
+                    $.param({
+                        language: data[fieldData.valueAttribute],
+                        next: window.location.href
+                    })
                 );
+                // Django will actually respond with a 302 redirect, but that would cause a page load during these
+                // unittests.  204 should work fine for testing.
                 AjaxHelpers.respondWithNoContent(requests);
                 FieldViewsSpecHelpers.expectMessageContains(view, 'Your changes have been saved.');
 
@@ -157,7 +162,10 @@ define(['backbone',
                     requests,
                     'POST',
                     '/i18n/setlang/',
-                    'language=' + data[fieldData.valueAttribute]
+                    $.param({
+                        language: data[fieldData.valueAttribute],
+                        next: window.location.href
+                    })
                 );
                 AjaxHelpers.respondWithError(requests, 500);
                 FieldViewsSpecHelpers.expectMessageContains(

lms/static/js/student_account/views/account_settings_fields.js

@@ -52,7 +52,8 @@
                 fieldTemplate: field_dropdown_account_template,
                 saveSucceeded: function() {
                     var data = {
-                        language: this.modelValue()
+                        language: this.modelValue(),
+                        next: window.location.href
                     };
 
                     var view = this;