Fix exchange_code_for_access_token by supplying Content-Type info

[yousong]

Otherwise, it will fail with the following error

Traceback (most recent call last):
  File "get_access_token.py", line 49, in <module>
access_token = api.exchange_code_for_access_token(code)
  File "/home/yousong/git-repo/python-instagram/instagram/oauth2.py", line 48, in exchange_code_for_access_token
return req.exchange_for_access_token(code=code)
  File "/home/yousong/git-repo/python-instagram/instagram/oauth2.py", line 116, in exchange_for_access_token
raise OAuth2AuthExchangeError(parsed_content.get("error_message", ""))
instagram.oauth2.OAuth2AuthExchangeError: You must provide a client_id

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