Fixes #37 with other default locale than english

Test in error with french default Locale:
org.junit.ComparisonFailure: expected:<[ne peut pas être vide]> but was:<[may not be empty]>
	at org.junit.Assert.assertEquals(Assert.java:115)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at org.springframework.samples.petclinic.model.ValidatorTests.emptyFirstName(ValidatorTests.java:39)