[enhancement] number_to_human_size helper to support 'unit' option

As I understand currently number_to_human_size helper supports bytes to nearest unit conversion. While using this helper I felt the need of an option ‘unit’ to specify the unit to return the conversion into.

For example,

number_to_human_size(File.size(abc.csv), unit: “MB”)

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This is feasible without much effort. In our application we have monkey patched it, as we need it a lot. I think this might be what a lot of developers need.

Maybe following will be better

number_to_human_size(File.size(abc.csv), unit: :mb)
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