added check in HttpBasicAuthenticator to see if Authorization header already exists from a previous Execute. Will no longer add header twice.
added check in HttpBasicAuthenticator to see if Authorization header already exists from a previous Execute. Will no longer add header twice.

fmmendo/RestSharp,mcintyre321/RestSharp-.NET-2.0-Fork,periface/RestSharp,paulcbetts/RestSharp,restsharp/RestSharp,jmartin84/RestSharp,mwereda/RestSharp,IsCoolEntertainment/unity-restsharp,dmgandini/RestSharp,dyh333/RestSharp,amccarter/RestSharp,mattwalden/RestSharp,rivy/RestSharp,uQr/RestSharp,mcintyre321/RestSharp-.NET-2.0-Fork,devinrader/RestSharpRTExperimental,jiangzm/RestSharp,who/RestSharp,paulcbetts/RestSharp,IsCoolEntertainment/unity-restsharp,RestSharp-resurrected/RestSharp,devinrader/RestSharpRTExperimental,cnascimento/RestSharp,dgreenbean/RestSharp,eamonwoortman/RestSharp.Unity,KraigM/RestSharp,felipegtx/RestSharp,PKRoma/RestSharp,SaltyDH/RestSharp,wparad/RestSharp,kouweizhong/RestSharp,chengxiaole/RestSharp,haithemaraissia/RestSharp,jmartin84/RestSharp,Haacked/RestSharp,mattleibow/RestSharp,benfo/RestSharp,lydonchandra/RestSharp,Haacked/RestSharp,wparad/RestSharp,huoxudong125/RestSharp,rucila/RestSharp