Fix typos for PRE and POST keys in the node_value_passed of not_exists operator

[quangtamle]

Description of Issue/Question

According to the code:

PRE and POST keys in the node_value_passed dict of not_exists operator are capitalized which are not synchronized (should be in small letters like the others)

I wonder whether those capital keys are on purpose or not. If not, can we fix those to small letters for unified format of node_value_passed dict ?

Setup

(Please provide relevant configs, xml snapshots and related files (Be sure to remove sensitive info).)
No need

Steps to Reproduce Issue

(Include debug logs if possible and relevant. Error trace would be helpful too)
No need

Versions Report

(Provided by running jsnapy --version. Please also mention python version.)
the current version

[chidanandpujar]

Fix is under review #414