Trailing slashes in target of dojo.store.JsonStore (Issue #16691)

Fixes issue #16691

Added property ```allowNoTrailingSlash``` to control the behaviour. Default is false.
If false, the previous behaviour is used, i.e. a trailing slash on ```target``` is expected.
If true, ```target``` is checked for a trailing slash. If none exists, then a slash appended to ```target```
If options.before is set and contains an id, then this id is also appended to the target URL.