Fixing issue with sorting which fails when different datatypes are given...

... in the sortby list.

Fixing sorting issue mentioned below :

If i have objects like these : 
`[{"1", 100}, {"1", 1}, {"1", 2}, {"1", 200}]`
If i sort the elements using both the columns then the result should be :
`[{"1", 1}, {"1", 2}, {"1", 100}, {"1", 200}]`
But in the current code the result that is displayed is :
`[{"1", 1}, {"1", 100}, {"1", 2}, {"1", 200}]`


this will call `_.keys(a)` on each iteration, you would want to cache this.

You may want to take a look at a relevant open pr, #1751 and issue #1768