Make sure Content-Type is the first added header.

I forgot that this was part of the design.  And it may be misguided.  It
may be that I should only concern myself with 'Content-' headers, and
ignore all others.  But what then about 'X-' headers associated with a
parts content?  So for now, I make sure Content-Type is first so that
the copy logic will get all headers added via the 'headers' keyword, as
documented.  To do this I had to add a feature to set_param (replace
keyword).  I also ended up simplifying the logic to add the headers,
once I realized that set_raw of course bypasses the duplicate header
check (yay for unit tests).