Fix fix fix fixWorker: Improve 'device' and 'real_mobile' normalisation
Fix fix fix fixWorker: Improve 'device' and 'real_mobile' normalisation

Various browsers (e.g. Opera 47 on Windows) are in the /browsers API as:
real_mobile=null, device=null

But then, once spawned are described as in /workers as
real_mobule=false, device=undefined

Normalise this as otherwise the workers are considered foreign
and the client keeps spawning more.

clarkbox/testswarm-browserstack,mzgol/testswarm-browserstack,clarkbox/testswarm-browserstack,mzgol/testswarm-browserstack