fix(dropdown): add missing TAB keyCode. closes #1577

[tmorehouse]

Description of PR:

Dropdowns missing keyCode definition for TAB. Also was not closing when focus (via TAB) moved out of the menu.

Closes #1577

---

PR checklist:

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fixes #xxxx[,#xxxx], where "xxxx" is the issue number)

If new features/enhancement/fixes are added or changed:

• Includes documentation updates
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (does it affect screen reader users or keyboard only users?)

If adding a new feature, or changing the functionality of an existing feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

PR titles should following the Conventional Commits naming convention

[codecov]

Codecov Report

Merging #2140 into dev will increase coverage by 0.02%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##            dev    #2140      +/-   ##
========================================
+ Coverage    65%   65.02%   +0.02%     
========================================
  Files       159      159              
  Lines      2960     2956       -4     
  Branches    813      811       -2     
========================================
- Hits       1924     1922       -2     
+ Misses      749      748       -1     
+ Partials    287      286       -1

Impacted Files	Coverage Δ
src/mixins/dropdown.js	7.24% <ø> (+0.1%)	⬆️
src/components/dropdown/dropdown.js	100% <ø> (ø)	⬆️
src/components/carousel/carousel-slide.js	61.53% <0%> (-5.13%)	⬇️
src/components/navbar/navbar-toggle.js	83.33% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 52c960b...6dd37a2. Read the comment docs.