fix(mouseup): do not reset when active element is in downshift

[kentcdodds]

What: do not reset when active element is in downshift

Why: Closes #356

How:

• Add util for checking whether a node is or contains something
• Use that util for both the event.target and the document.activeElement
• Use that instead of the input

Checklist:

• Documentation N/A
• Tests
• Ready to be merged
  
• Added myself to contributors table N/A
  

[kentcdodds]

This is the last PR before I create the next branch. All future work will start there until it's merged. Hopefully we'll get that merged before the week is out.

[codecov-io]

Codecov Report

Merging #383 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #383   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         338    337    -1     
  Branches       90     90           
=====================================
- Hits          338    337    -1

Impacted Files	Coverage Δ
src/utils.js	100% <100%> (ø)	⬆️
src/downshift.js	100% <100%> (ø)	⬆️

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