The HTTP/1.1 RFC[0] specifies that the host header MUST be included in each request.

   >A client MUST include a Host header field in all HTTP/1.1 request
   >messages . If the requested URI does not include an Internet host
   >name for the service being requested, then the Host header field MUST
   >be given with an empty value. An HTTP/1.1 proxy MUST ensure that any
   >request message it forwards does contain an appropriate Host header
   >field that identifies the service being requested by the proxy. All
   >Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)
   >status code to any HTTP/1.1 request message which lacks a Host header
   >field.

[0] https://tools.ietf.org/html/rfc2616#page-128