Clear _downKeys on dispatch

[stevenlundy]

This is a possible solution to the issue I filed at jaywcjlove/react-hotkeys#29

This change starts from an empty _downKeys array in case a key wasn't cleared (e.g. no key up is fired on mac for cmd+up)

It will work given:

• Only key combos with modifiers and up to one key are supported by this library
• The dispatch function rebuilds _downKeys to include correct modifiers

There are other changes that could be made given that we start from an empty _downKeys array, but this seemed like a good start and doesn't break any tests.

[coveralls]

Pull Request Test Coverage Report for Build 209

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 81.179%

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Totals
Change from base Build 208:	-0.2%
Covered Lines:	194
Relevant Lines:	208

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #122 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   93.23%   93.26%   +0.03%     
==========================================
  Files           1        1              
  Lines         207      208       +1     
==========================================
+ Hits          193      194       +1     
  Misses         14       14

Impacted Files	Coverage Δ
dist/hotkeys.js	93.26% <100%> (+0.03%)	⬆️

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[jaywcjlove]

@stevenlundy I tested it and it didn't solve the problem, which caused some functions to fail.