Make objectContaining generic?

[studds]

objectContaining is currently declared as:

function objectContaining(expectedValue: Object): any

Couple of drawbacks:

• You lose any sort of typechecking as soon as you wrap an object in objectContaining
• I've got the https://palantir.github.io/tslint/rules/no-unsafe-any/ linting rule turned on, so the linter complains every time I use this.

If the signature was changed to:

function objectContaining<T extends object>(expectedValue: T): T;

both these issues would be resolved, at the cost of complaining about partial objects, so bit of a tradeoff. Personally, I'd prefer to have typechecking in my tests and explicitly cast when required, rather than have everything mapped to any...

I'd be happy to do a PR if there was support for this idea... thoughts?