Add CheckReturnValue annotation to peekJson. (#736)

* Add CheckReturnValue annotation to peekJson.

* Suppress test warning.

moshi/src/main/java/com/squareup/moshi/JsonReader.java

@@ -497,7 +497,7 @@ public abstract class JsonReader implements Closeable {
    *   jsonReader.nextInt() // Returns 456, reader contains 789 and ].
    * }</pre>
    */
-  public abstract JsonReader peekJson();
+  @CheckReturnValue public abstract JsonReader peekJson();
 
   /**
    * Returns a <a href="http://goessner.net/articles/JsonPath/">JsonPath</a> to

moshi/src/test/java/com/squareup/moshi/JsonReaderTest.java

@@ -38,6 +38,7 @@ import static org.junit.Assert.fail;
 import static org.junit.Assume.assumeTrue;
 
 @RunWith(Parameterized.class)
+@SuppressWarnings("CheckReturnValue")
 public final class JsonReaderTest {
   @Parameter public JsonCodecFactory factory;