Fix uncaught error when parsing rest headers, document Started this work as better docs for rest response format. But I found 406 error response was not being tested. Also there was no error for bad Content-Type. In rest.py fix uncaught exceptions due to invalid Accept or Content-Type headers. If Content-type is valid but not application/json return code 415. Document use of accept header (was only shown in examples) and support for q parameter. Describe using .xml and .json extensions to select return format for testing from browser (where setting accept header is a problem). Document 406 error code return. Document 415 error code return and acceptable content types. Previously only doc was in examples. Set up tests for 406 and 415 error codes.

--- a/CHANGES.txt	Sun Dec 27 15:04:02 2020 -0500
+++ b/CHANGES.txt	Fri Jan 01 14:14:34 2021 -0500
@@ -63,6 +63,9 @@
 - fixed some issues when generating translations. Use mappings and
   named format parameters so translators can move substituted tokens
   in tranlsations.
+- in rest interface, fix uncaught exceptions when parsing invalid
+  Content-Type and Accept headers. Document response formats more
+  fully in doc/rest.txt.
 
 Features:
 - issue2550522 - Add 'filter' command to command-line