Header name containing underscore about npoi.mapper

[donnytian]

below chars will be removed from the name to ensure it's a valid property name. source code:

// Default chars that will be removed when mapping by column header name.
private static readonly char[] DefaultIgnoredChars =
{'`', '~', '!', '@', '#', '$', '%', '^', '&', '*', '-', '_', '+', '=', '|', ',', '.', '/', '?'};

if you want to keep the underscore, just write this

// remove underscore
mapper.IgnoredNameChars = {'`', '~', '!', '@', '#', '$', '%', '^', '&', '*', '-', '+', '=', '|', ',', '.', '/', '?'};

but yes, underscore is valid in a property name, I will consider removing it from the default array.