This might be embarrassing to admit. But it did take some time to find this option. The option for disabling fiddler from updating/correcting the content-length header. This is especially handy if you need to test how your webserver reacts to wrongly assigned content-lengths. At least that was what I needed it for.
Go to the options tab - if you are not already in the composer tab then you have to open that first.
Uncheck the checkbox beside "Fix Content-Length header".
That is all! Fiddler will no longer fix the content-length header for you. Remember to reenable this when you are done. It is quite annoying having to set this header yourself.
PS. You can also make fiddler not follow redirects in this tab.