use explicit type for mock body

see bblimke/webmock#427
replace webmock stubs with faraday stubs

to correctly handle the serialized JSON
Merge pull request #79 from koenpunt/fix-webmock-invalid-body

use explicit type for mock body
add application/hal+json to 'Accept' header
change default content-type to application/hal+json

add custom middleware for json+hal