deep-extend: fix object type checking bug

[erfanium]

typeof o === "object" doesn't always return expected result (e.g: with array, null, regexp, ....).

[coveralls]

Coverage increased (+0.001%) to 99.744% when pulling 70cb6bb on erfanium:master into 3a2378f on icebob:master.

[erfanium]

@icebob
I think this code is better

[icebob]

OK, but I feel a little lack when the source has an empty object {} property, in this case, the pointer will be copied instead of copy the object. But I hope it won't cause a problem.