concat should try to convert_types
Problem:
A data set is distributed over multiple csv files generated by different instances. Some of these instances log the 'header' line multiple times (when restarted). Currently I'm cleaning out these spurious header lines and doing pd.concat. 
But this results in the dtype of columns being object as columns from files with spurious header lines fail the lib.maybe_convert_numeric in the parser. Hence, although pd.concat is being given clean data, the concatenated data ends up having columns with object dtype

Possible Solution:
Call parser._convert_types or equivalent from pd.concat for object type columns
