ServiceBusMessage testing fix

[hallvictoria]

_servicebus.py is no longer an abstract class. ServiceBusMessage can now be tested similarly to QueueMessage and HTTPRequest in unit tests.

Fixes Azure/azure-functions-python-worker#1321

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (717bf06) 91.25% compared to head (6baa94a) 92.96%.

Files	Patch %	Lines
azure/functions/_servicebus.py	94.73%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #192      +/-   ##
==========================================
+ Coverage   91.25%   92.96%   +1.71%     
==========================================
  Files          57       56       -1     
  Lines        3406     3027     -379     
  Branches      847      286     -561     
==========================================
- Hits         3108     2814     -294     
+ Misses        220      132      -88     
- Partials       78       81       +3     

Flag	Coverage Δ
unittests	92.96% <94.87%> (+1.74%)	⬆️

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