Given the header name and the value provided by the application
program, return the (name, value) that should be stored in the model.
The name is returned unchanged. If the input value has a 'name'
attribute and it matches the name ignoring case, the value is returned
unchanged. Otherwise the name and value are passed to header_factory
method, and the resulting custom header object is returned as the
value. In this case a ValueError is raised if the input value contains
CR or LF characters.