Replace Dictionary.Add with Item property so that parameters may be o…

[ramsay]

…verwritten without thowing exceptions

[codecov-io]

Codecov Report

Merging #22 into master will not change coverage.
The diff coverage is 40%.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   40.06%   40.06%           
=======================================
  Files           6        6           
  Lines         302      302           
  Branches       19       19           
=======================================
  Hits          121      121           
  Misses        178      178           
  Partials        3        3

Impacted Files	Coverage Δ
Telesign/MessagingClient.cs	0% <0%> (ø)	⬆️
Telesign/ScoreClient.cs	0% <0%> (ø)	⬆️
Telesign/VoiceClient.cs	0% <0%> (ø)	⬆️
Telesign/RestClient.cs	80.66% <100%> (ø)	⬆️

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