requireFilesExist rule should be case sensitive

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-enforcer-plugin</artifactId>
    <version>3.0.0-M3</version>
    <executions>
        <execution>
            <id>enforce-files-exist</id>
            <goals>
                <goal>enforce</goal>
            </goals>
            <configuration>
                <rules>
                    <requireFilesExist>
                        <files>
                            <file>src/main/resources/i18n_de.properties</file>
                        </files>
                    </requireFilesExist>
                </rules>
                <fail>true</fail>
            </configuration>
        </execution>
    </executions>
</plugin>