Added ability to set headers on statsapi.game() requests

[tracerrx]

Added data.headers with a constant API HEADERS. Updated the statsapi.game() requests in

data/dates.py
data/game.py
data/standings.py
data/teams.py
data/unifiorms.py

Need Brian Ward to take a look at the main game.py requests and replace his testing_params

I'm honestly not sure how to test that were actually sending the correct headers

[WardBrian]

For the testing params, you should be able to leave them with just | after the params, like

live_data = statsapi.get("game", {"gamePk": self.game_id, "fields": API_FIELDS} | testing_params , data.headers.API_HEADERS)

[tracerrx]

There are also some headers specified in headlines.py that are hard coded and not part of the mlb api request.