BUG/MEDIUM: http: fetch "base" is not compatible with set-header

The sample fetch function "base" makes use of the trash which is also
used by set-header/add-header etc... everything which builds a formated
line. So we end up with some junk in the header if base is in use. Let's
fix this as all other fetches by using a trash chunk instead.

This bug was reported by Baptiste Assmann, and also affects 1.5.