GeezerVison

This commit includes None value in merged output dict for keys that only exist in second input dict (also while nested).
The original logic is preserving a[key] value if b[key] is None, this remains unchanged.

Added test cases for this behavior.

Context:
This function is used while getting Config.settings.
As per CONFIGURATION.md pylsp.rope.ropeFolder can be a null value.
When configuration is received from the client, it is correctly parsed to None value, thus the
config._settings["rope"]["ropeFolder] = None.
Once merged the output settings["rope"] dict didn't contain key "ropeFolder".

I believe that merge dict was behaving correctly if "rope" key didn't exist however when workspace/didChangeConfiguration got sent and "rope" key did exist with dict containing "extensionModules" list, in my case, the "ropeFolder" was completely ignored by merge_dicts

This will fix #664 and I believe #622

Fyi @osiewicz