I contain the value portion of a MIME-compatible header.

I must be only initialized with the value and not the field name. E.g. in processing
Subject: This is the subject
the MIMEHeaderValue should be given only 'This is the subject'

For traditional non-MIME headers, the complete value returned for mainValue and paramaters returns an empty collection.

For MIME headers, both mainValue and parameters are used.

	printing
	asHeaderValue printOn:

	accessing	top
	mainValue
	mainValue:
	parameterAt:put:
	parameters
	parameters:

	printing	top
	asHeaderValue
	printOn: Append to the argument, aStream, a sequence of characters that identifies the receiver.

	instance creation	top
	forField:fromString: Create a MIMEHeaderValue from aString. How it is parsed depends on whether it is a MIME specific field or a generic header field.
	fromMIMEHeader: This is the value of a MIME header field and so is parsed to extract the various parts
	fromTraditionalHeader: This is a traditional non-MIME header (like Subject:) and so should be stored whole