Allow tabs in header value, fixes #2263

Author: alexeyzimarev

src/RestSharp/Parameters/HeaderParameter.cs

@@ -53,7 +53,6 @@ static bool IsInvalidHeaderString(string stringValue) {
         // ReSharper disable once ForCanBeConvertedToForeach
         for (var i = 0; i < stringValue.Length; i++) {
             switch (stringValue[i]) {
-                case '\t':
                 case '\r':
                 case '\n':
                     return true;