http_parameters:http_convert

Conversion of an HTTP form value. First tries the multifile hook http:convert_parameter/3 and next the built-in checks.

Arguments:

`Option`	- List as provided with the parameter
`FieldName`	- Name of the HTTP field (for better message)
`ValueIn`	- Atom value as received from HTTP layer
`ValueOut`	- Possibly converted final value

Errors

- type_error(Type, Value)