"FieldValues..." in REST API gives double-encoded field names
As reported by Shai Petel in github (https://github.com/SharePoint/sp-dev-docs/issues/1141), column names with non-ASCII characters or "special" characters (i.e. those that get encoded) are being double encoded by the REST API in the FieldValues... properties (this includes FieldValuesAsText, FieldValuesAsHtml, and FieldValuesForEdit).
For example, create a column called "Français" in a list with Title "Test" and call the following REST request: /api/web/lists/getbytitle('Test')/items?$expand=FieldValuesAsText,FieldValuesAsHtml,FieldValuesForEdit. in the response given the field's internal name is properly shown in the field values as "Franx00e7ais", but is shown as "Franx005fx00e7x005f_ais" in all of the "FieldValues..." properties.
This makes it very difficult for the developer since you must always look for the doubly-encoded property name in the "FieldValues..." properties instead of the singly-encoded name found elsewhere in the schema. They won't match up.