Custom XML Key improvements

[caronc]

Description:

Related issue (if applicable): #908

Improvements to the XML Key Handling (and Test Cases) based on some minor bugs found in the JSON version.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@908-custom-xml-service-improvements

# Test out the changes with the following command:
# The below renames the Message Stanza to Body
apprise -t "Test Title" -b "Test Message" \
  "xml://localhost/:Message:Body

[codecov-commenter]

Codecov Report

Patch coverage is 100.00% of modified lines.

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Files Changed	Coverage
apprise/plugins/NotifyXML.py	100.00%

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