Fix route creation when format is a blank string

Commit bff61ba, while reducing allocations, caused a regression when an empty
format is passed to a route.

This can happen in cases where you're using an anchor tag, for example:
`https://example.com/parent/575256966.#child_1032289285`.

Because of this change `format` was getting sent in
`parameterized_parts` when previously it was not included. This resulted
in blank `format`'s being returned as `.` when if there was an extension
included it would be `.extension`. Since there was no extension this
caused incorrect URL's.

The test shows this would result in `/posts/show/1.` instead of
`/posts/show/1` which causes bad urls since the format is not present.

actionpack/lib/action_dispatch/journey/formatter.rb

@@ -33,7 +33,7 @@ def generate(name, options, path_parameters, parameterize = nil)
           defaults       = route.defaults
           required_parts = route.required_parts
           parameterized_parts.keep_if do |key, value|
-            defaults[key].nil? || value.to_s != defaults[key].to_s || required_parts.include?(key)
+            (defaults[key].nil? && value.present?) || value.to_s != defaults[key].to_s || required_parts.include?(key)
           end
 
           return [route.format(parameterized_parts), params]

actionpack/test/controller/url_for_integration_test.rb

@@ -158,6 +158,7 @@ def setup
 
       ['/posts/ping',[    { :controller => 'posts', :action => 'ping' }]],
       ['/posts/show/1',[  { :controller => 'posts', :action => 'show', :id => '1' }]],
+      ['/posts/show/1',[  { :controller => 'posts', :action => 'show', :id => '1', :format => '' }]],
       ['/posts',[         { :controller => 'posts' }]],
       ['/posts',[         { :controller => 'posts', :action => 'index' }]],
       ['/posts/create',[  { :action => 'create' }, {:day=>nil, :month=>nil, :controller=>"posts", :action=>"show_date"}, '/blog']],