Fix regression in isObject (crashed w/ null values).

[halflings]

Accessing a column that has null values crashes in the latest version of danfojs, e.g for a dataframe:

df = DataFrame([{b: 5}, {a: 12, b:3}, {b:2}])

Accessing df["a"] would crash on isObject looking for a constructor for the null value.

It's odd because I found a previous pull request fixing this:
#208
Not sure why this regression was introduced.

I also tried adding a test to catch such regressions in the future but noticed all columns' tests are commented out in:
tests/core/frame.js

Is that a temporary measure? Having all of those commented out might let such errors slip in in future releases.

[risenW]

Accessing a column that has null values crashes in the latest version of danfojs, e.g for a dataframe:

df = DataFrame([{b: 5}, {a: 12, b:3}, {b:2}])

Accessing df["a"] would crash on isObject looking for a constructor for the null value.

It's odd because I found a previous pull request fixing this: #208 Not sure why this regression was introduced.

I also tried adding a test to catch such regressions in the future but noticed all columns' tests are commented out in: tests/core/frame.js

Is that a temporary measure? Having all of those commented out might let such errors slip in in future releases.

Thanks for the fix! I'll review and merge. Regarding the commented-out test, It must have been a mistake, I'll uncomment them and ensure they pass now.