Fix struct key malformed error message

[AlexGx]

Current

produces an inconsistent (poorly formatted) error messages when passing non-atom keys to a struct:

iex> %URI{"port" => 80}
error: invalid key "port" for struct, struct keys must be atoms, got:

Expected

message like:

error: invalid key for struct, struct keys must be atoms, got: "port"

Changes

• fix error format
• tighten test

[josevalim]

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