NLogLoggingConfiguration support custom section names

[anellomancante]

Root LoggingConfigurationElement gets now created with a constant "NL…og" name (as expected by LoggingConfigurationParser) when the actual IConfigurationSection has a different key. Resolves #487

[snakefoot]

Would it be possible to create a small unit-test, by creating a special memoryConfig ?

[anellomancante]

I refactored all the tests to take custom section name into account.

[codecov-io]

Codecov Report

Merging #488 (fbf0e04) into master (b6955c5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #488   +/-   ##
=======================================
  Coverage   81.25%   81.25%           
=======================================
  Files          15       15           
  Lines        1296     1296           
  Branches      223      223           
=======================================
  Hits         1053     1053           
  Misses        165      165           
  Partials       78       78           

Impacted Files	Coverage Δ
...ensions.Logging/Config/NLogLoggingConfiguration.cs	92.34% <100.00%> (ø)

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[snakefoot]

I like unit-tests to be simple, and to only focus on one thing. Please undo the use of [Theory] and just make one new unit-test

[snakefoot]

@anellomancante Thank you for the contribution and the unit-test!