Add class and test to explore issue #20.

src/test/java/com/fatboyindustrial/gsonjavatime/Issue20.java

@@ -0,0 +1,37 @@
+package com.fatboyindustrial.gsonjavatime;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.time.ZoneOffset;
+
+/**
+ * Test case from {@code https://github.com/gkopff/gson-javatime-serialisers/issues/20}.
+ */
+@SuppressWarnings({ "FieldCanBeLocal", "unused" })         // used reflectively
+public class Issue20
+{
+  private static final Gson gson = Converters.registerAll(new GsonBuilder()).create();
+
+  private final Instant date;
+  private final LocalDateTime date2;
+  private final LocalDate date3;
+  private final Instant date4;
+
+  public Issue20(final Instant date)
+  {
+    this.date = date;
+    this.date2 = LocalDateTime.ofInstant(date, ZoneId.of("UTC"));
+    this.date3 = this.date2.toLocalDate();
+    this.date4 = this.date3.atStartOfDay().toInstant(ZoneOffset.UTC);
+  }
+
+  public String toJSON()
+  {
+    return gson.toJson(this);
+  }
+}

src/test/java/com/fatboyindustrial/gsonjavatime/Issue20Test.java

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+package com.fatboyindustrial.gsonjavatime;
+
+import org.junit.Test;
+
+import java.time.Instant;
+
+import static org.hamcrest.Matchers.is;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Test for {@code https://github.com/gkopff/gson-javatime-serialisers/issues/20}.
+ */
+public class Issue20Test
+{
+  /**
+   * Tests that the serialisation produces ISO format strings.
+   */
+  @Test
+  public void test()
+  {
+    final Instant instant = Instant.parse("2017-06-08T22:11:28.566Z");
+
+    final Issue20 issue = new Issue20(instant);
+    final String expected =
+        "{\"date\":\"2017-06-08T22:11:28.566Z\"," +
+        "\"date2\":\"2017-06-08T22:11:28.566\"," +
+        "\"date3\":\"2017-06-08\"," +
+        "\"date4\":\"2017-06-08T00:00:00Z\"}";
+
+    assertThat(issue.toJSON(), is(expected));
+  }
+}