Support spy calls in assertions

[mantoni]

Purpose (TL;DR) - mandatory

This allows to pass a call as the first argument to sinon assertions,
e.g. assert.calledWith(spy.firstCall, 123). This is supported by Sinon
but wasn't working with this library because of the argument validateion
being too restrictive.

Solution - optional

This change removes the typeof "function" check in verifyFakes and checks if the given argument supports calledWith instead of checking for getCall.

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 66

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 92.982%

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Files with Coverage Reduction	New Missed Lines	%
referee-sinon.js	1	92.98%

Totals
Change from base Build 63:	-0.2%
Covered Lines:	122
Relevant Lines:	125

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💛 - Coveralls

[mroderick]

👍

[mantoni]

Released in v4.1.0.