http: deprecate legacy array header notation
Ever since commit 316e2833, flat object header notation has been preferred over array-in-array notation, ex:
[['Content-Type', 'text/plain'], ['X-Data', 'hello world']]
This commit officially deprecates this functionality. This frees us up for further optimizations and cleaner code, removing the spaghetti-like residue of this five-year-old feature.