Fix: bootmixin should use super.component() to get binding key

[mschnee]

There is a small misbehavior in boot.mixin.ts, where upon invoking this.component(ComponentClass), mountComponentBooters will assume "components.${class.name}" to be the binding key, even if another key was specified.

This presents itself in Applications where components marked with @bind() using a custom name, and attempting to use them:

export const COMPONENT_KEY = BindingKey.create<MyComponent>('org.my.namespace.MyComponent');
@bind({
  tags: {[ContextTags.KEY]: COMPONENT_KEY},
  scope: BindingScope.SINGLETON,
})
export class MyComponent implements Component, LifeCycleObserver {
}

import { MyComponent } from '@myScope/my-library';
export class MyApplication extends BootMixin(ServiceMixin(RestApplication)) {
  constructor() {
    this.component(MyComponent);
    // Error: The key 'components.MyComponent' is not bound to any value in context
  }
}

This PR addresses this, by implementing a similar fix to what was done in repository.mixin.ts.

See also #5477

Checklist

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• [n/a] API Documentation in code was updated
• [n/a] Documentation in /docs/site was updated
• [n/a] Affected artifact templates in packages/cli were updated
• [n/a] Affected example projects in examples/* were updated

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[raymondfeng]

@mschnee Thanks. Please run npm run prettier:fix to fix the formatting issue - https://travis-ci.com/github/strongloop/loopback-next/jobs/338658227

[mschnee]

@raymondfeng lint complete. Looks good, minus the MacOS test failing.

[jannyHou]

Thank you @mschnee good catch, I left a few suggestions. LGTM 👍
I triggered a new build for the macos test.

[mschnee]

@jannyHou @raymondfeng I've updated the tests and added documentation, as suggested :)

[mschnee]

I see that I added some code that has no coverage- I will update this when I'm back at my computer after the holiday weekend :)

[bajtos]

Thank you for the pull request, @mschnee! ❤️

I quickly skimmed through the changes and they look good at high level. Please try to get approval from other people who commented in this pull request before you proceed to landing.

[jannyHou]

Thanks! LGTM

[raymondfeng]

@mschnee Ping. Can we fix up the minor issues commented by @bajtos and land it?

[mschnee]

@raymondfeng good morning! I'm working on this now :)