Make with_path_root() update existing headers rather than overwrite them

[karandeep-johar]

When using the Python SDK with a team-based access token and accessing a user's root namespace you would typically perform the following steps:

team_dbx = dropbox.DropboxTeam("access token")
user_dbx = team_dbx.as_user("member id")

get root namespace via get_current_account()

user_with_path_root_dbx = user_dbx.with_path_root(PathRoot.root("root ns"))

user_with_path_root_dbx.files_list_folder(path="")
...
When doing this, the Select-User header is overwritten inside with_path_root().

Headers before call to with_path_root():

{'Dropbox-API-Select-User': 'dbmid:AACJlzwHrjpPqqnHys__AnsTo9CLNUou6Ek'}
Headers after calling with_path_root():

{'Dropbox-API-Path-Root': '{".tag": "root", "root": "1337955185"}'}
Headers we want after calling with_path_root():

{'Dropbox-API-Path-Root': '{".tag": "root", "root": "1337955185"}', 'Dropbox-API-Select-User': 'dbmid:AACJlzwHrjpPqqnHys__AnsTo9CLNUou6Ek'}
with_path_root() should modify the headers rather than overwriting them.