add Basic auth header to the request

[bulkan]

This is needed as per the documentation

curl -v https://api.sandbox.paypal.com/v1/identity/openidconnect/tokenservice \
-u "{clientId}:{secret}" \
-d "grant_type=authorization_code
    &code={authorizationCode}
    &redirect_uri=http://example.com/myapp/return.php"

The curl example passes on Basic Auth

fixes #26

[tomwhipple]

@bulkan Thanks for the contribution. @avidas Please review & merge if acceptable.

[avidas]

Looks good, merging.