Currency list we are passing in config as string can be atoms about gringotts HOT 3 CLOSED

Atoms are best suited for keys (in a map or keyword), as far as I've read.
We'll have to convert these atoms to upcase strings when making requests to the gateway anyways. There's a thread on elixirforum discussing serialisation and atoms.

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As there is limit on atoms count in erlang. link

• Though using atom for currency/country code will not increase atom count at great extent. But huge project may consider huge atom count for using library.
• Also some gateway are using atoms for keys in the post body creation. So should we use string or atom there.

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This is now obsolete because of #62 and #71.
It's the Money lib's responsibility to provide a sane/nice interface to create Money objects.

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