Constructs a basic authentication header from the user name and password specified, and adds it to the request headers.
It is used when the variable shouldPresentCredentialsBeforeChallenge is set to YES.
Usage
ODPRequest *request=[ODPRequest requestWithURL:[NSURL URLWithString:endpointUrl]];
[request addBasicAuthenticationHeaderWithUsername:"smpuser" andPassword:@"s3puser"];
[request startSynchronous];