Bug fixed.

# Objective
The bug related to #160 is fixed.

# Problem
Although `convert_object` convert `1.0e-12` to `1.0000e-12`, it convert `1e-12` to `0` when all column values is match with the regular expression "[1-9]+e-[1-9]+".

# Changes
Before applying `convert_object`, all values are tried to convert to float type.