replaceValue crashes when the target property has no value or separator #82

public class TestProperties {
    @Test
    void testPutReplaceMissingSepOrValue() throws IOException, URISyntaxException {
        final String given = "key1=val1\n" +
                "key2=\n" +
                "key3";
        final String expected = "key1=value1\n" +
                "key2=value2\n" +
                "key3=value3";

        Properties p = Properties.loadProperties(new StringReader(given));
        p.put("key1", "value1");
        p.put("key2", "value2");
        p.put("key3", "value3");

        StringWriter sw = new StringWriter();
        p.store(sw);
        assertThat(sw.toString()).isEqualTo(expected);
    }
}