Change how the column data for player_game_log is parsed

[avadhanij]

Filing an enhancement as discussed. Currently, for parsing the response of https://stats.nba.com/stats/playergamelogs, the column names are hard coded, and this makes it brittle and harder to maintain with changes made to output data format.

Since the column names are supplied in the spec, it's better to take that, and use it directly.