Align schema validation with specification for objects

[feenst]

Only an object should check for required properties based on the specification:

For each of these types, there are keywords that only apply to those types. For example, numeric types have a way of specifying a numeric range, that would not be applicable to other types.

See also comments in this issue.

[philsturgeon]

I'll be happy to merge this ASAP if you can accompany it with a test.

[feenst]

I'll be happy to merge this ASAP if you can accompany it with a test.

Test added

[coveralls]

Pull Request Test Coverage Report for Build 2320955039

• -1 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.027%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/validators/spec.js	3	4	75.0%
<!--	Total:	3	4	75.0%	-->

Totals
Change from base Build 2320977531:	0%
Covered Lines:	265
Relevant Lines:	273

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2320986666

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 94.027%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/validators/spec.js	3	4	75.0%
<!--	Total:	3	4	75.0%	-->

Totals
Change from base Build 2320977531:	-0.5%
Covered Lines:	265
Relevant Lines:	273

---

💛 - Coveralls