emit_once.diff

diff --git a/lib/Mojo/Content.pm b/lib/Mojo/Content.pm
index 50255e8..add63d1 100644
--- a/lib/Mojo/Content.pm
+++ b/lib/Mojo/Content.pm
@@ -295,7 +295,7 @@ sub _parse_until_body {
   $self->{raw_size} += length($chunk //= '');
   $self->{pre_buffer} .= $chunk;
   $self->_parse_headers if ($self->{state} ||= 'headers') eq 'headers';
-  $self->emit('body') if $self->{state} ne 'headers' && !$self->{body}++;
+  $self->emit_once('body') if $self->{state} ne 'headers';
 }
 
 1;
diff --git a/lib/Mojo/EventEmitter.pm b/lib/Mojo/EventEmitter.pm
index 794decf..09007cb 100644
--- a/lib/Mojo/EventEmitter.pm
+++ b/lib/Mojo/EventEmitter.pm
@@ -22,6 +22,8 @@ sub emit {
   return $self;
 }
 
+sub emit_once { $_[0]{once}{$_[1]}++ ? shift : shift->emit(@_) }
+
 sub has_subscribers { !!@{shift->{events}{shift()} || []} }
 
 sub on {
@@ -135,6 +137,13 @@ Subscribe to L</"error"> event.
 
 Emit event.
 
+=head2 emit_once
+
+  $e = $e->emit_once('foo');
+  $e = $e->emit_once('foo', 123)
+
+Emit event only once.
+
 =head2 has_subscribers
 
   my $bool = $e->has_subscribers('foo');
diff --git a/lib/Mojo/Message.pm b/lib/Mojo/Message.pm
index 244aa7d..51b1ff0 100644
--- a/lib/Mojo/Message.pm
+++ b/lib/Mojo/Message.pm
@@ -89,7 +89,7 @@ sub extract_start_line {
 sub finish {
   my $self = shift;
   $self->{state} = 'finished';
-  return $self->{finished}++ ? $self : $self->emit('finish');
+  return $self->emit_once('finish');
 }
 
 sub fix_headers {