Fix deprecation warning with Content-Type header without modification

Author: ydakuka

lib/jsonapi/rails.rb

@@ -37,7 +37,7 @@ def self.install!
     # @return [NilClass]
     def self.add_errors_renderer!
       ActionController::Renderers.add(:jsonapi_errors) do |resource, options|
-        self.content_type ||= Mime[:jsonapi]
+        self.content_type = Mime[:jsonapi] if self.media_type.nil?
 
         many = JSONAPI::Rails.is_collection?(resource, options[:is_collection])
         resource = [resource] unless many
@@ -90,7 +90,7 @@ def self.add_errors_renderer!
     # @return [NilClass]
     def self.add_renderer!
       ActionController::Renderers.add(:jsonapi) do |resource, options|
-        self.content_type ||= Mime[:jsonapi]
+        self.content_type = Mime[:jsonapi] if self.media_type.nil?
 
         JSONAPI_METHODS_MAPPING.to_a[0..1].each do |opt, method_name|
           next unless respond_to?(method_name, true)