[chroma-js] fix bug with Scale.colors.

[NaridaL]

Please fill in this template.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes:

> c = require('chroma-js')
> c.scale(['white', c('red')]).colors()
[ '#ffffff', '#ff0000' ]
> c.scale(['white', c('red')]).colors(3)
[ '#ffffff', '#ff8080', '#ff0000' ]
> c.scale(['white', c('red')]).colors(undefined)
[ '#ffffff', '#ff0000' ]
> c.scale(['white', c('red')]).colors(undefined, null)
[ Color { _rgb: [ 255, 255, 255, 1, _clipped: false ] },
  Color { _rgb: [ 255, 0, 0, 1, _clipped: false ] } ]
> c.scale(['white', c('red')]).colors(undefined, undefined)
[ Color { _rgb: [ 255, 255, 255, 1, _clipped: false ] },
  Color { _rgb: [ 255, 0, 0, 1, _clipped: false ] } ]
> c.scale(['white', c('red')]).colors(undefined, 'css')
[ 'rgb(255,255,255)', 'rgb(255,0,0)' ]

colors(2) and colors(2, undefined) lead to different results ><

• Increase the version number in the header if appropriate. Not necessary.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }. N/A

[NaridaL]

The bug was that, as the format parameter was marked as optional for all overloads, TS would pick the first one when null or undefined was passed, resulting in string[] cannot be assigned to Color[]

[typescript-bot]

@NaridaL Thank you for submitting this PR!

🔔 @invliD @mpacholec - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

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[typescript-bot]

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