Add optional feature to fill nulls in Arrow::Array#to_narray

[mrkn]

Currently Arrow::Array#to_narray raises an error when the receiver array has nulls.
I'd like to make this method to fill all the nulls by a specific value.

My API proposal is here:

nary = ary.to_narray(fillna: 0)  # filling nulls by 0
nary = ary.to_narray(fillna: Float::NAN)  # filling nulls by NaN

[kou]

The feature should exist inArrow::Array. But not yet for now. https://issues.apache.org/jira/browse/ARROW-1567

Example:

arrow_array.fill_null(0).to_narray