Fix :mongo adapter to clean up newly created collection

[mtsmfm]

ref: DatabaseCleaner/database_cleaner#646

[codecov]

Codecov Report

Merging #6 into master will increase coverage by 4.85%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master        #6      +/-   ##
===========================================
+ Coverage   95.14%   100.00%   +4.85%     
===========================================
  Files           6         6              
  Lines         103        91      -12     
===========================================
- Hits           98        91       -7     
+ Misses          5         0       -5     

Impacted Files	Coverage Δ
lib/database_cleaner/mongo/truncation.rb	100.00% <100.00%> (+12.50%)	⬆️
spec/database_cleaner/mongo/truncation_spec.rb	100.00% <100.00%> (ø)
spec/support/database_helper.rb	100.00% <100.00%> (ø)

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[etagwerker]

@mtsmfm Thanks for submitting this PR! It looks good to me, but I don't know enough about this adapter to approve it.

@botandrose could you review it as well? thanks!

[botandrose]

@mtsmfm Thank you for this PR, and your work on the mongo adapter in general. With this, I think it its ready for v2.0!

@etagwerker This one was pretty interesting. Turns out the mongo adapter has been basically unusable for eight years! See original PR for more context: DatabaseCleaner/database_cleaner#646