Add colon to safe characters

[malthe]

This matches the output from generating a SAS token using Azure portal.

[codecov-io]

Codecov Report

Merging #682 (67eeb84) into master (4306898) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #682   +/-   ##
=======================================
  Coverage   87.05%   87.05%           
=======================================
  Files          52       52           
  Lines        5308     5308           
  Branches      570      570           
=======================================
  Hits         4621     4621           
  Misses        515      515           
  Partials      172      172           

Impacted Files	Coverage Δ
...e-blob/azure/storage/blob/sharedaccesssignature.py	72.58% <100.00%> (ø)

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