Fix an error caused by combining %@ with other placeholders

Fixes #234

lib/twine/placeholders.rb

@@ -24,7 +24,7 @@ module Twine
       # %@ -> %s
       value = convert_twine_string_placeholder(input)
 
-      number_of_placeholders = number_of_twine_placeholders(input)
+      number_of_placeholders = number_of_twine_placeholders(value)
 
       return value if number_of_placeholders == 0
 

test/test_placeholders.rb

@@ -79,6 +79,10 @@ class PlaceholderTest < TwineTest
         to_android("some %d second %2$f")
       end
     end
+
+    def test_complicated_float_placeholders
+      assert_equal "%1$.0f%2$s (apparent: %3$.0f)", to_android("%.0f%@ (apparent: %.0f)")
+    end
   end
 
   class FromAndroid < PlaceholderTest