Added failing test for json_escape striping quotation marks

Expanded test coverage for html_escape and json_escape
Fixed a long-standing bug in `json_escape` that strips quotation marks
Use lower case letters in unicodes sequences to match the new encoder's output
Added \u2028 \u2029 to json_escape
Also move html_esacpe regex to a constant (see 9d25af60)
Merge pull request #13073 from chancancode/json_escape

Fixed json_escape (again)