Mockito.spy cannot work on final objects.

Fixes #16.

Author: Capstan

src/test/java/com/github/fge/jackson/JsonNodeReaderTest.java

@@ -50,7 +50,7 @@ public final class JsonNodeReaderTest
     public void streamIsClosedOnRead()
         throws IOException
     {
-        final Supplier<InputStream> supplier = provideInputStream("[]");
+        Supplier<InputStream> supplier = provideInputStream("[]");
         final InputStream in = spy(supplier.get());
         final JsonNode node = new JsonNodeReader().fromInputStream(in);
         verify(in).close();
@@ -61,7 +61,7 @@ public void streamIsClosedOnRead()
     public void readerIsClosedOnRead()
         throws IOException
     {
-        final Supplier<Reader> supplier = provideReader("[]");
+        Supplier<Reader> supplier = provideReader("[]");
         final Reader reader = spy(supplier.get());
         final JsonNode node = new JsonNodeReader().fromReader(reader);
         assertEquals(node, new ObjectMapper().readTree(supplier.get()));