Adds a value to the header field.
- iOS 7.0+
- macOS 10.9+
- tvOS 9.0+
- watchOS 2.0+
- Xcode 8.0+
The value for the header field.
The name of the header field. In keeping with the HTTP RFC, HTTP header field names are case insensitive.
This method provides the ability to add values to header fields incrementally. If a value was previously set for the specified field, the supplied value is appended to the existing value using the appropriate field delimiter (a comma).
Certain header fields are reserved (see Reserved HTTP Headers). Do not use this method to change such headers.