Fix stubbing function objects

[mantoni]

Purpose (TL;DR) - mandatory

Currently sinon.stub(function () {}) throws TypeError: Attempted to wrap undefined property undefined as function. Sinon should either throw a meaningful error explaining why a function is not a valid argument, or treat functions like any other object. I can't think of a reason why we shouldn't support this.

Fixes #1967

How to verify - mandatory

1. Check out this branch
2. npm install
3. npm t

Checklist for author

• npm run lint passes
• References to standard library functions are cached.

[coveralls]

Pull Request Test Coverage Report for Build 2783

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 94.526%

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Files with Coverage Reduction	New Missed Lines	%
sinon/stub.js	1	96.55%

Totals
Change from base Build 2780:	0.004%
Covered Lines:	1664
Relevant Lines:	1730

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💛 - Coveralls

[mantoni]

I don't know what's wrong with the CircleCI build and won't have time to look into it. Everything works fine on Travis.

[mantoni]

Released in v7.2.3.