Modify @Condition for detecting existing message bundle resource

The problem with the old code is that it worces a ResourceBundle to
initialize with the default encoding (and that is then cached in the
JDK) during @Condition evaluation (so before the encoding is known).

Includes test for swedish messages

Fixes gh-1228

spring-boot-autoconfigure/src/test/resources/test/swedish.properties

@@ -0,0 +1 @@
+foo=Some text with some swedish öäå!