Support mongo < 3.4 when collection didn't have type attribute.

[dooman87]

[codecov-io]

Codecov Report

Merging #5 into master will decrease coverage by 0.11%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
- Coverage   81.06%   80.95%   -0.12%     
==========================================
  Files           4        4              
  Lines         169      168       -1     
==========================================
- Hits          137      136       -1     
  Misses         18       18              
  Partials       14       14

Impacted Files	Coverage Δ
migrate.go	71.92% <100%> (-0.25%)	:arrow_down:

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[xakep666]

Thank you! I've used your commits as base for v0.3.0.