Reduce actual for match and matchJson

[mantoni]

When asserting plain objects using match or matchJson, set the "actual"
object to an excerpt only containing the properties specified by the
matcher. If the original "actual" object is used, the diff produced by
test runners shows all properties that are not defined in the matcher as
missing. With this change, only properties that are actually compared
are shown.

How to verify - mandatory

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

Also the matchJson demo was enhanced. Run it with npm run demo.

Checklist for author

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

[coveralls]

Pull Request Test Coverage Report for Build 145

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 95.933%

---

Files with Coverage Reduction	New Missed Lines	%
assertions/match-json.js	2	90.48%

Totals
Change from base Build 143:	0.1%
Covered Lines:	553
Relevant Lines:	570

---

💛 - Coveralls

[mroderick]

Looks good 👍

[mantoni]

Released in v2.5.0.