THRIFT-2070 Improper `HexChar' and 'HexVal' implementation in TJSONProtocol.cs

Patch: Yousong Zhou
diff --git a/lib/csharp/src/Protocol/TJSONProtocol.cs b/lib/csharp/src/Protocol/TJSONProtocol.cs
index f583781..3b27d30 100644
--- a/lib/csharp/src/Protocol/TJSONProtocol.cs
+++ b/lib/csharp/src/Protocol/TJSONProtocol.cs
@@ -407,6 +407,7 @@
 			}
 			else if ((ch >= 'a') && (ch <= 'f'))
 			{
+				ch += 10;
 				return (byte)((char)ch - 'a');
 			}
 			else
@@ -428,6 +429,7 @@
 			}
 			else
 			{
+				val -= 10;
 				return (byte)((char)val + 'a');
 			}
 		}