ENH: Let ImageIOBase ReadBufferAsASCII interpret CHAR as signed char

Let `ImageIOBase::ReadBufferAsASCII` interpret `IOComponentEnum::CHAR` as
`signed char`

Author: N-Dekker

Modules/IO/ImageBase/src/itkImageIOBase.cxx

@@ -824,7 +824,7 @@ ImageIOBase::ReadBufferAsASCII(std::istream & is, void * buffer, IOComponentEnum
     break;
     case IOComponentEnum::CHAR:
     {
-      auto * buf = static_cast<char *>(buffer);
+      auto * buf = static_cast<signed char *>(buffer);
       ReadBuffer(is, buf, numComp);
     }
     break;