fix(b-nav-item-dropdown): clicking toggle a second time should close menu (closes #3707)

[tmorehouse]

Describe the PR

b-nav-item-dropdown wasn't closing the menu on second click of the toggler, unlike b-dropdown

One Issue was that there was a check for toggle event defaultPrevented (native event), and b-link when HREF is '#' prevents default to stop scroll to top behaviour, which prevents b-nav-item-dropdown from closing the menu.

Removed the check for defaultPrevented in dropdown mixin.

The other issue was that focusInHandler() used this.$refs.toggle to check wether the event target is contained. this.$refs.toggle can be a component in some places. Using the computed toggler property for the check solved the problem.

Closes #3707.

PR checklist

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

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

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

• No
• Yes (please describe)

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 (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, fix typos, chore: fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages.

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

• Includes documentation updates (including updating the component's package.json for slot and event changes)
• 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? Clickable items should be in the tab index, etc.)

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

[codecov]

Codecov Report

Merging #3706 into dev will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #3706      +/-   ##
==========================================
- Coverage   99.29%   99.29%   -0.01%     
==========================================
  Files         223      223              
  Lines        4378     4377       -1     
  Branches     1237     1235       -2     
==========================================
- Hits         4347     4346       -1     
  Misses         25       25              
  Partials        6        6

Impacted Files	Coverage Δ
src/components/nav/nav-item-dropdown.js	100% <ø> (ø)	⬆️
src/mixins/dropdown.js	100% <100%> (ø)	⬆️

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