`nvm_has` matches shell functions and aliases,
but downloads now run via `command`, which skips them
- a `curl` shell function with no curl binary on the PATH would select the curl path and fail with exit 127,
instead of falling back to an available wget executable.
The new `nvm_has_executable` helper resolves names the same way `command` does, so downloader selection and execution agree.
The wget path passed `NVM_AUTH_HEADER` as the raw header line
(e.g. `--header "Bearer secret-token"`),
omitting the `Authorization:` header name that the curl path includes.
Per the documented usage
(`NVM_AUTH_HEADER="Bearer secret-token"`) the value is the credential,
so wget was sending a malformed header.
Prefix it with `Authorization: ` to match the curl path.
Jordan Harband