A fix for missed client_secret in auth_code (You must provide a client_secret error)

[radzserg]

Client secret is not passed anymore.
ueberauth/oauth2@7d5ae39
This causes

%OAuth2.Client{authorize_url: "https://api.instagram.com/oauth/authorize/",
client_id: "40b734125eb44a2caf3b96ce70439fe2",
client_secret: "2e3c6ad2fff14709a792e8800cbed39c", headers: [], params: %{},
redirect_uri: "http://localhost:4000/instagram/token",
site: "https://api.instagram.com", strategy: Elixtagram.OAuthStrategy,
token: %OAuth2.AccessToken{access_token: nil, expires_at: nil,
other_params: %{"code" => 400,
"error_message" => "You must provide a client_secret",
"error_type" => "OAuthException"}, refresh_token: nil,
token_type: "Bearer"}, token_method: :post,
token_url: "https://api.instagram.com/oauth/access_token"}

Check out this thread ueberauth/oauth2#64
I did the same update for exlitagram.

[coveralls]

Coverage increased (+0.003%) to 99.415% when pulling dfc7ff4 on radzserg:master into c4087f5 on Zensavona:master.

[Zensavona]

hey mate, sorry about the delay - this looks good, I'm merging this now and updating some packages + cutting a release tonight :)

Thanks for the contribution!