Increased test coverage

[hschovanec-usgs]

Increased test coverage for transfer classes using mock.

Also made some edits to the code to fix bugs:

• In the City class columns() was updated to columns to get the list of dataframe columns.
• The HistoricTime class's strftime method didn't have an option for 'force', but it is required later in the method. Force was added as an optional argument.
• The cancel method in the EmailSender class was updated to account for no max_bcc specified.

closes #126

[codecov-io]

Codecov Report

Merging #126 (2b793bd) into master (4ed0ef8) will increase coverage by 15.38%.
The diff coverage is 96.15%.

@@             Coverage Diff             @@
##           master     #126       +/-   ##
===========================================
+ Coverage   74.70%   90.09%   +15.38%     
===========================================
  Files          36       36               
  Lines        4547     4551        +4     
===========================================
+ Hits         3397     4100      +703     
+ Misses       1150      451      -699     

Impacted Files	Coverage Δ
impactutils/time/timeutils.py	64.68% <ø> (+3.57%)	⬆️
impactutils/transfer/ftpsender.py	73.68% <ø> (+58.77%)	⬆️
impactutils/transfer/securesender.py	76.92% <ø> (+63.58%)	⬆️
impactutils/transfer/emailsender.py	83.65% <95.45%> (+70.38%)	⬆️
impactutils/io/smcontainers.py	88.67% <100.00%> (+88.67%)	⬆️
impactutils/mapping/city.py	82.66% <100.00%> (+4.66%)	⬆️
impactutils/time/ancient_time.py	92.50% <100.00%> (+64.84%)	⬆️
impactutils/rupture/quad_rupture.py	99.40% <0.00%> (+0.39%)	⬆️
... and 9 more

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