BUG: MINCImageIO should use signed char for CHAR and MI_TYPE_BYTE

`MINCImageIO::Write` should use `signed char` when the pixel component type is
`IOComponentEnum::CHAR`. Both `IOComponentEnum::CHAR` and `MI_TYPE_BYTE` specify
a _signed_ type, whereas the default `char` might be unsigned (for example, when
using GCC compiler option `-funsigned-char`).

Author: N-Dekker

Modules/IO/MINC/src/itkMINCImageIO.cxx

@@ -1396,7 +1396,7 @@ MINCImageIO::Write(const void * buffer)
       break;
     case IOComponentEnum::CHAR:
       volume_data_type = MI_TYPE_BYTE;
-      get_buffer_min_max<char>(buffer, buffer_length, buffer_min, buffer_max);
+      get_buffer_min_max<signed char>(buffer, buffer_length, buffer_min, buffer_max);
       break;
     case IOComponentEnum::USHORT:
       volume_data_type = MI_TYPE_USHORT;