Allow the Dispatcher to set is_behind_proxy for Core::Request:

Currently, Core::Request's is_behind_proxy is set by Core::App
whenever a new Core::Context is provided (using a trigger, no less).

There are various problems with this:
* The App shouldn't care about is_behind_proxy
* The attribute shouldn't be set every time, since it's global
* Core::Context is unrelated to this

Instead, we made behind_proxy a global variable (as done by
GH #590) with its own trigger. Then it is set on instantiation by
Core::Dispatcher.

Then we can remove Core::App's _init_for_context method completely.

GH #590 approaches this issue in an entirely different and better
way, by suggesting we simply correct the environment before all
of this happens using a middleware.

For now we're just doing enough to untangle Core::Context more.

lib/Dancer2/Core/App.pm

@@ -254,8 +254,6 @@ has context => (
 sub setup_context {
     my ( $self, $ctx ) = @_;
 
-    $self->_init_for_context($ctx);
-
     for my $type ( @{ $self->supported_engines } ) {
         my $attr   = "${type}_engine";
         my $engine = $self->$attr or next;
@@ -419,15 +417,6 @@ sub _init_hooks {
     );
 }
 
-sub _init_for_context {
-    my ($self) = @_;
-
-    $self->has_request or return;
-
-    $self->request->is_behind_proxy(1)
-      if $self->setting('behind_proxy');
-}
-
 sub supported_hooks {
     qw/
       core.app.before_request

lib/Dancer2/Core/Dispatcher.pm

@@ -116,8 +116,9 @@ sub build_request {
     # If we have an app, send the serialization engine
     my $engine  = $app->engine('serializer');
     my $request = Dancer2::Core::Request->new(
-          env        => $env,
-        ( serializer => $engine ) x!! $engine,
+          env             => $env,
+          is_behind_proxy => Dancer2->runner->config->{'behind_proxy'} || 0,
+        ( serializer      => $engine ) x!! $engine,
     );
 
     # Log deserialization errors

lib/Dancer2/Core/Request.pm

@@ -369,8 +369,9 @@ has body_is_parsed => (
 );
 
 has is_behind_proxy => (
-    is      => 'rw',
+    is      => 'ro',
     isa     => Bool,
+    lazy    => 1,
     default => sub {0},
 );
 

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;
+        },
     } },
 );