Don't transform the scale specified by the user

[oterral]

This PR fixes a regression introduced by this one #4302 . The default mulitplier scale should be used only when the user hasn't specified a scale and when the defualt image is used.

[bartvde]

@oterral any chance of adding a unit test? TIA

[oterral]

I'd love it. But can't compile on my mac I have an error:

/Users/kogisiwi/Documents/00_GIT/oterral-ol3/node_modules/metalsmith/lib/index.js:190
Metalsmith.prototype.build = unyield(function*(){
                                             ^
SyntaxError: Unexpected token *
    at Module._compile (module.js:439:25)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)

[bartvde]

try with node 4.2.2

[tsauerwein]

See #4356 (comment)

curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.29.0/install.sh | bash
nvm install 4
nvm alias default stable

[oterral]

Thanks guys you're awesome !!! Test is coming.

[oterral]

Tests done but build failed caused by #4384

[bartvde]

thanks @oterral