Send correct XML for get_settings

[wiegandm]

The get_settings method previously sent get_schedules instead of the correct GMP command.

This PR provides the unit test used for confirming this bug, the bug fix, a CHANGELOG entry and a unit test for the get_setting (singular) method.

[codecov]

Codecov Report

Merging #80 into master will increase coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   57.84%   58.11%   +0.27%     
==========================================
  Files          10       10              
  Lines        2218     2218              
==========================================
+ Hits         1283     1289       +6     
+ Misses        935      929       -6

Impacted Files	Coverage Δ
gvm/protocols/gmpv7.py	53.53% <100%> (+0.34%)	⬆️

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[codecov]

Codecov Report

Merging #80 into master will increase coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   57.84%   58.11%   +0.27%     
==========================================
  Files          10       10              
  Lines        2218     2218              
==========================================
+ Hits         1283     1289       +6     
+ Misses        935      929       -6

Impacted Files	Coverage Δ
gvm/protocols/gmpv7.py	53.53% <100%> (+0.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 63c7f9b...d815e2e. Read the comment docs.