MRCReader: assume file is little endian unless marked othewise

Some MRC file writers don't follow the standard and fail to fill in the
endianness field.  So unless the file is marked as big endian, assume it is
little endian.

IO/Image/vtkMRCReader.cxx

@@ -226,9 +226,11 @@ int vtkMRCReader::RequestInformation(vtkInformation* vtkNotUsed(request),
       return 0;
       }
     this->Internals->stream->read((char*)&this->Internals->header, sizeof(mrc_file_header));
-    if (this->Internals->header.stamp[0] == 'D')
-      // Officially, the next character should be 'A', but this appears to vary
-      // between implementations, so I only test the first one
+    if (this->Internals->header.stamp[0] != ((char)17))
+    // This is what the big-endian MRC files are supposed to look like.  I don't have one to
+    // test with though.  However, if it does not look like that, assume it is little endian.
+    // There are some non-conformant programs that don't correctly fill in this field, and
+    // assuming little endian is safer.
       {
       vtkByteSwap::Swap4LERange(&this->Internals->header,24);
       vtkByteSwap::Swap2LERange(&this->Internals->header.creatid,1);
@@ -388,7 +390,13 @@ void vtkMRCReader::ExecuteDataWithInformation(vtkDataObject *vtkNotUsed(output),
   inOffsets[1] = this->Internals->header.nx * numComponents;
   inOffsets[2] = this->Internals->header.ny * this->Internals->header.nx
               * numComponents;
-  bool fileIsLittleEndian = (this->Internals->header.stamp[0] == 'D');
+
+  // This is what the big-endian MRC files are supposed to look like.  I don't have one to
+  // test with though.  However, if it does not look like that, assume it is little endian.
+  // There are some non-conformant programs that don't correctly fill in this field, and
+  // assuming little endian is safer.
+  bool fileIsLittleEndian = (this->Internals->header.stamp[0] != ((char)17));
+
   ByteSwapFunction byteSwapFunction = getByteSwapFunction(vtkType,fileIsLittleEndian);
   switch (vtkType)
     {