Address comments

Author: wschin

src/Microsoft.ML.Data/DataLoadSave/Text/TextLoader.cs

@@ -978,7 +978,9 @@ private bool HasHeader
         /// <param name="dataSample">Allows to expose items that can be used for reading.</param>
         /// <param name="allowSparse">Whether the file can contain numerical vectors in sparse format.</param>
         /// <param name="allowQuoting">Whether the file can contain numerical vectors in sparse format.</param>
-        public TextLoader(IHostEnvironment env, Column[] columns, bool hasHeader = false, char separatorChar = '\t', IMultiStreamSource dataSample = null, bool allowSparse = false, bool allowQuoting = false)
+        public TextLoader(IHostEnvironment env, Column[] columns, bool hasHeader = DefaultArguments.HasHeader,
+            char separatorChar = DefaultArguments.Separator, IMultiStreamSource dataSample = null,
+            bool allowSparse = DefaultArguments.AllowSparse, bool allowQuoting = DefaultArguments.AllowQuoting)
             : this(env, MakeArgs(columns, hasHeader, new[] { separatorChar }, allowSparse, allowQuoting), dataSample)
         {
         }

src/Microsoft.ML.Data/DataLoadSave/Text/TextSaver.cs

@@ -24,7 +24,7 @@ internal sealed class TextSaver : IDataSaver
     {
         internal static class DefaultArguments
         {
-            internal const char Separator = '\t';
+            internal const string Separator = "tab";
             internal const bool ForceDense = false;
             internal const bool OutputSchema = true;
             internal const bool OutputHeader = true;
@@ -34,7 +34,7 @@ internal static class DefaultArguments
         public sealed class Arguments
         {
             [Argument(ArgumentType.AtMostOnce, HelpText = "Separator", ShortName = "sep")]
-            public string Separator = DefaultArguments.Separator.ToString();
+            public string Separator = DefaultArguments.Separator;
 
             [Argument(ArgumentType.AtMostOnce, HelpText = "Force dense format", ShortName = "dense")]
             public bool Dense = DefaultArguments.ForceDense;