Http Server: set a default type in path holder to be json

For most cases the return type of a handler is json, this adds a default
behavior so if the path holder is osVersion and the handler function is
f it will be possible to do:

osVersion.set_handler(f);
or
osVersion.set_handler("txt",f);

Keeping the simplified option when type is supplied but allows to ommit
it for the common case.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Http Server: Move the parameters object to the request

The parameter map, holds path parameters. For example the if a url
matching rule is defined as /file/{path} and a url is /file/etc/hosts
the param object will hold an entry for "path" => "/etc/hosts".

To simplify the API the parameters object was moved to the request
object. It can only be calculated by the routes, as it depends on
specific handler (ie. /file/etc/{path} and /file/{path} will have
different value in the path entry)

For that the routes will get the request as non const reference and
will use the parameters object that is now in the request.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>