Remove special treatment of "host" header

[mseri]

As correctly pointed out by @anuragsoni, the RFC says that order of header fields isn't significant within a well-behaved server.

The order in which header fields with differing field names are
received is not significant. However, it is good practice to send
header fields that contain control data first, such as Host on
requests and Date on responses, so that implementations can decide
when not to handle a message as early as possible. A server MUST NOT
apply a request to the target resource until the entire request

Since this introduces unnecessary complexity and unnecessary extra functions in the http package header module, I am removing the special treatment in this PR.

Users that want to enforce the ordering of the headers can do it with the same type of overhead directly in their code and can take 0d252ec as inspiration. Using Header.to_list and of_list in their code it is also safer than what we do here, in case the implementation details of the header module change.

[mseri]

Ping @anuragsoni @avsm

[mseri]

In this case, we are favouring simplicity of the implementation over best practices (see also the relevant issue: #934)

[mseri]

Moreover users that want to enforce Host as the first header can always add it as first header in the Headers parameter.