Fix setPlaceholder to update this.options.placeholder

[marexandre]

Fix setPlaceholder so that when setPlaceholder is called afterwards it return updated placeholder string

Before

Example of the bug and what this PR tries to fix:

const geocoder = new MapboxGeocoder({
    accessToken: mapboxgl.accessToken,
    mapboxgl: mapboxgl,
    placeholder: 'Test'
});

geocoder.getPlaceholder();
// returns Test"

geocoder.setPlaceholder('New string');

geocoder.getPlaceholder();
// returns  "Test"  and not the updated placeholder text

After

const geocoder = new MapboxGeocoder({
    accessToken: mapboxgl.accessToken,
    mapboxgl: mapboxgl,
    placeholder: 'Test'
});

geocoder.getPlaceholder();
// returns  "Test"

geocoder.setPlaceholder('New string');

geocoder.getPlaceholder();
// returns  "New string" 

• [x] briefly describe the changes in this PR
• [x] write tests for all new functionality
• [x] run npm run docs and commit changes to API.md
• [x] update CHANGELOG.md with changes under master heading before merging

[marexandre]

If I wanted to have this change in v4 of this library, what would be the recommendations to add a similar fix?

[marexandre]

CI is failing for the following reason - which does not seem to be related to changes in this PR...

[marexandre]

@stepankuzmin thank you for taking a look at this changes - rant the tests locally and all passed 🎉 How should we proceed with merging this PR? Anything else I should do or missed?