Fix 1445: make stubbing of static function properties possible

[raulmatei]

Purpose (TL;DR) - mandatory

Fix issue #1445 related to stubbing function static properties by adding an extra check for the object type to be a function

[coveralls]

Coverage remained the same at 94.957% when pulling 452981c on raulmatei:fix-1445-sandbox-stubbing-static-function-property-throws-error into 25f3eeb on sinonjs:master.

[lucasfcosta]

LGTM!
Thanks for this @raulmatei.

[mroderick]

I think this looks like a new MINOR version. What say you @sinonjs/core?

[lucasfcosta]

@mroderick I think this looks more like a PATCH, because this should have been possible before. AFAIK this is what we expected from the sandbox API since the beginning of its design.

According to the docs:

The sandbox stub method can also be used to stub any kind of property. This is useful if you need to override an object’s property for the duration of a test, and have it restored when the test completes.

[mroderick]

Cheers @lucasfcosta!

I've published this change as v2.3.3