Fix import to use a block to configure the http post

[jshow]

For discussion, no tests written as yet - the branch is working for my purposes presently

Background

Import is broken. #106

The MixPanel import api requires the API SECRET to be passed in basic authentication

This PR adds a configuration block to the HTTP Post object, so basic_auth can be configured through a block

in host app config/initializers/mixpanel.rb

Mixpanel.config_http_request do |http_request|
  http_request.basic_auth "#{ENV['MIXPANEL_API_SECRET']}", ''
end

[andrewpurcell]

Hi @jshow, thanks for opening this PR. We can't respond immediately but we'll get back to you after we investigate.

[DataGreed]

@andrewpurcell that's quite a long investigation you conduct, guys. Really long for a paid service.

[reinaris]

@andrewpurcell any change to have a look at this?