Headers fixes

This PR fixes some problems detected while testing almodovar 1.0.0.pre using webmock for headers matching, that to the best of my knowledge are:
- Session headers hadn't been taken into account
- Content type header was provided as understood by patron using `:content_type` symbol instead of the proper header name  `Content-Type` (https://github.com/bebanjo/almodovar/blob/v0.9.3/lib/almodovar/single_resource.rb#L18)