Sparse arr test and extra commenting to _.isEqual
spec NaN equals Object(NaN)
allow falsey contexts in lookupIterator (fixes #1674)
Merge pull request #1673 from megawac/equal-comments

Add comments and sparse array test for _.isEqual
Update description of _.isEmpty.

The HTML documentation did not make it clear how _.isEmpty works for strings and arrays vs. objects
Merge pull request #1672 from scottaj/update-isEmpty-docs

Update description of _.isEmpty.