Fixed issue where CsvConfiguration.NewLine was being set when value i…

…s null in constructor causing IsNewLine to be true.

src/CsvHelper/Configuration/CsvConfiguration.cs

@@ -279,7 +279,7 @@ public CsvConfiguration(
 			MemberTypes = memberTypes ?? MemberTypes;
 			MissingFieldFound = missingFieldFound ?? MissingFieldFound;
 			Mode = mode ?? Mode;
-			NewLine = newLine ?? NewLine;
+			if (newLine != null) NewLine = newLine;
 			PrepareHeaderForMatch = prepareHeaderForMatch ?? PrepareHeaderForMatch;
 			ProcessFieldBufferSize = processFieldBufferSize ?? ProcessFieldBufferSize;
 			Quote = quote ?? Quote;