When `try_into_http_request` was used with an invalid base URI the code
was panicking. Acording to `http::request::Builder` documentation,
`headers_mut` returns `None` if the builder contains errors, which was
the case when an invalid URI was provided.
The new version only sets the additional headers in case that there are
no errors on the builder, preventing the panic. The conversion will
return an error when the builder is consumed on `body`.
