[BUGFIX] quoteJSvalue double encodes forward slash

The new JavaScript encoder using json_encode also encodes the forward
slash to unicode: \u00F2.

This creates problems, since json_encode already 'escapes' the forward
slash with a backslash.

This patch removes the ecoding of the forward slash from quoteJSvalue

Unit tests have been adjusted to match the output of json_encode:
* forward slash is escaped with a backslash

Change-Id: Ibc19f69c0b68f5bbfa67a39750884dcc030f38cb
Resolves: #55499
Releases: 6.2
Reviewed-on: https://review.typo3.org/27183
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Tested-by: Daniel Siepmann
Reviewed-by: Frans Saris
Tested-by: Frans Saris