http: set state.infilesize when sending formposts

Without it set, we would unwillingly triger the "HTTP error before end
of send, stop sending" condition even if the entire POST body had been
sent (since it wouldn't know the expected size) which would
unnecessarily log that message and close the connection when it didn't
have to.

Reported-by: Matt McClure
Bug: https://curl.haxx.se/mail/archive-2019-02/0023.html
Closes #3624