tool_cb_hdr: Fix --remote-header-name with schemeless URL

- Move the existing scheme check from tool_operate.

In the case of --remote-header-name we want to parse Content-disposition
for a filename, but only if the scheme is http or https. A recent
adjustment 0dc4d8e4 was made to account for schemeless URLs however it's
not 100% accurate. To remedy that I've moved the scheme check to the
header callback, since at that point the library has already determined
the scheme.

Bug: https://github.com/curl/curl/issues/760
Reported-by: Kai Noda