Fix issue 764 to handle edge case in one hot encoding transformation

[paullo0106]

Proposed changes

This PR fixes #764 by assigning X_new to the first non-null transformed column X_col rather than only the first column.

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[paullo0106]

@jeongyoonlee I feel logically speaking, this PR makes sense to improve the code (which comes from Kaggler repo and we can modify things there too if needed).

However, I'm curious about which dataset was used by craftsliu that x.fillna(NAN_INT).map(self.label_encoders[i]).fillna(0) in _transform_col() would return a null value and then therefore throw an error in transform() function