http: fix memory leak exposed in concurrent servers

http: fix memory leak exposed in concurrent servers

First off, this memory leak DOES NOT affect Unicorn itself.
Unicorn allocates the HttpParser once and always reuses it
in every sequential request.

This leak affects applications which repeatedly allocate a new
HTTP parser.  Thus this bug affects _all_ deployments of
Rainbows! and Zbatery.  These servers allocate a new parser for
every client connection.

I misread the Data_Make_Struct/Data_Wrap_Struct documentation
and ended up passing NULL as the "free" argument instead of -1,
causing the memory to never be freed.

From README.EXT in the MRI source which I misread:
> The free argument is the function to free the pointer
> allocation.  If this is -1, the pointer will be just freed.
> The functions mark and free will be called from garbage
> collector.