Fix flowrate test

[jaekwon]

NOTE: PR is against jae/dbprefix, not develop.

I discovered this bug while running the test...

[codecov-io]

Codecov Report

Merging #165 into jae/dbprefix will increase coverage by 3.79%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           jae/dbprefix    #165      +/-   ##
===============================================
+ Coverage          58.7%   62.5%   +3.79%     
===============================================
  Files                40      59      +19     
  Lines              2492    4296    +1804     
===============================================
+ Hits               1463    2685    +1222     
- Misses              916    1394     +478     
- Partials            113     217     +104

Impacted Files	Coverage Δ
log/nop_logger.go	0% <0%> (ø)
log/tracing_logger.go	66.66% <0%> (ø)
merkle/simple_tree.go	62.16% <0%> (ø)
merkle/simple_map.go	71.79% <0%> (ø)
log/logger.go	0% <0%> (ø)
log/tm_logger.go	45.16% <0%> (ø)
pubsub/query/query.go	63.67% <0%> (ø)
pubsub/query/query.peg.go	64.38% <0%> (ø)
flowrate/util.go	60% <0%> (ø)
log/tm_json_logger.go	100% <0%> (ø)
... and 9 more

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[jaekwon]

Related: mxk/go-flowrate#3