Make behind_proxy a global var

Conditionally apply BehindProxy middleware if behind_proxy is set.

Making the "behind_proxy" option a global trigger that updates the
Dancer 2::Runner object when
  set behind_proxy => 1;
is run so the reverse proxy middleware can be conditionally wrapped
around applications psgi coderef.

We now 'use' the middleware directly rather than letting Placl::Builder
load it for us. May allow earlier version of Placl to be used as a dependency.

lib/Dancer2/Core/App.pm

@@ -413,9 +413,6 @@ sub _init_for_context {
 
     return if !defined $self->context;
     return if !defined $self->context->request;
-
-    $self->context->request->is_behind_proxy(1)
-      if $self->setting('behind_proxy');
 }
 
 sub supported_hooks {

lib/Dancer2/Core/Role/ConfigReader.pm

@@ -89,6 +89,11 @@ has global_triggers => (
             my ( $self, $handler ) = @_;
             Dancer2->runner->config->{'apphandler'} = $handler;
         },
+
+        behind_proxy => sub {
+            my ( $self, $flag ) = @_;
+            Dancer2->runner->config->{'behind_proxy'} = $flag;
+        },
     } },
 );
 

lib/Dancer2/Core/Runner.pm

@@ -6,7 +6,10 @@ use Dancer2::Core::MIME;
 use Dancer2::Core::Types;
 use Dancer2::Core::Dispatcher;
 use HTTP::Server::PSGI;
-use Plack::Builder qw();
+
+use Plack::Middleware::Head;
+use Plack::Middleware::Conditional;
+use Dancer2::Middleware::BehindProxy;
 
 with 'Dancer2::Core::Role::ConfigReader';
 
@@ -240,9 +243,13 @@ sub psgi_app {
         return $response;
     };
 
-    my $builder = Plack::Builder->new;
-    $builder->add_middleware('Head');
-    return $builder->wrap($psgi);
+    $psgi = Plack::Middleware::Conditional->wrap(
+        $psgi,
+        builder   => sub { Dancer2::Middleware::BehindProxy->wrap($_[0]) },
+        condition => sub { $self->config->{'behind_proxy'} },
+    );
+    $psgi = Plack::Middleware::Head->wrap($psgi);
+    return $psgi;
 }
 
 sub print_banner {