Verification of dtypes of columns of X_row* is same that self.X

[salmuz]

Hello, I want to contribute by fixing two different bugs that are related to the usage of Ligthgbm. 

1. NaN values in the category columns (which can cause an exception if we want to sorted(...))

       '<' not supported between instances of 'str' and 'float'

2. Preserve the right dtypes of columns of X (dataframe) so that that Ligthm predict(..) function doesn't throw errors.

[oegedijk]

cool, thanks! tests are passing, but please have a look at my comments and see if you can add a few test cases for this new function...

[salmuz]

Hello, I will do the requested changes as soon as possible (the next week). Thanks