xmlimporter.add_object(): Exclusion of HasEncoding target objects sometimes fails for 'Default JSON'

[DrMatthiasArnold]

Describe the bug

When loading node sets with user specific data types, the supported encoding rules are provided via HasEncoding references which point to an object with display names "Default Binary", "Default XML", "Default JSON".
(for further information OPC 10000-6: UA Part 6: Mappings)
Since these referenced objects typically don't have parent objects within the namespace, the xmlimporter.add_object() method excludes such objects from further checks.
However, due to a misspelling "Default Json" instead of "Default JSON" this exclusion does not work as expected and loading node sets could fail with an error.

Corrected version of code (lines xmlimporter.py 358 - 360):

# do not verify these nodes because some nodesets contain invalid elements
if obj.displayname != 'Default Binary' and obj.displayname != 'Default XML' and obj.displayname != 'Default JSON':
    res[0].StatusCode.check()

To Reproduce

Specific node set still to be published.

Version

Python-Version:

opcua-asyncio Version (master branch & update):