send text/plain as content-type instead of an empty string

Author: gustavosaume

lib/grape/api.rb

@@ -492,7 +492,7 @@ def add_head_not_allowed_methods
         not_allowed_methods = %w(GET PUT POST DELETE PATCH HEAD) - methods
         not_allowed_methods << "OPTIONS" if self.class.settings[:do_not_route_options]
         not_allowed_methods.each do |bad_method|
-          @route_set.add_route( proc { [405, { 'Allow' => allow_header, 'Content-Type' => '' }, []]}, {
+          @route_set.add_route( proc { [405, { 'Allow' => allow_header, 'Content-Type' => 'text/plain' }, []]}, {
             :path_info      => path_info,
             :request_method => bad_method
           })

spec/grape/api_spec.rb

@@ -403,7 +403,7 @@ def subject.enable_root_route!
         "example"
       end
       put '/example'
-      last_response.headers['Content-Type'].should eql ''
+      last_response.headers['Content-Type'].should eql 'text/plain'
     end
 
     it 'adds an OPTIONS route that returns a 204 and an Allow header' do