Please add chaining mutators

It's a common form to want to return a newly-constructed object with all the values set into it already, by having mutator methods that return the invocant, so you can easily add more. In such a style you could write:

return HTTP::Response->new( 200 )
    ->set_content_type( "text/plain" )
    ->set_content( "Hello, world" );

As it currently stands, the existing mutator methods like ->header, ->content, etc.. do not return the invocant, thus such a style is impossible and one must use a temporary:

my $resp = HTTP::Response->new( 200 );
$resp->content_type( "text/plain" );
$resp->content( "Hello, world" );
return $resp;

Start with the HTTP::Response mutator methods named in the issue, including header, content, set_content_type, and set_content. Check how their current return values are specified or tested, then update the relevant mutators so chaining returns the invocant and verify the example construction style works.