Fix TypeAdapter.toJson throwing AssertionError for custom IOException

gson/src/main/java/com/google/gson/TypeAdapter.java

@@ -216,7 +216,7 @@ public final String toJson(T value) {
     try {
       toJson(stringWriter, value);
     } catch (IOException e) {
-      throw new AssertionError(e); // No I/O writing to a StringWriter.
+      throw new JsonIOException(e);
     }
     return stringWriter.toString();
   }

gson/src/test/java/com/google/gson/TypeAdapterTest.java

@@ -2,6 +2,7 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.fail;
 
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
@@ -26,6 +27,38 @@ public void testNullSafe() throws IOException {
     assertNull(adapter.fromJson("null"));
   }
 
+  /**
+   * Tests behavior when {@link TypeAdapter#write(JsonWriter, Object)} manually throws
+   * {@link IOException} which is not caused by writer usage.
+   */
+  @Test
+  public void testToJson_ThrowingIOException() {
+    final IOException exception = new IOException("test");
+    TypeAdapter<Integer> adapter = new TypeAdapter<Integer>() {
+      @Override public void write(JsonWriter out, Integer value) throws IOException {
+        throw exception;
+      }
+
+      @Override public Integer read(JsonReader in) throws IOException {
+        throw new AssertionError("not needed by this test");
+      }
+    };
+
+    try {
+      adapter.toJson(1);
+      fail();
+    } catch (JsonIOException e) {
+      assertEquals(exception, e.getCause());
+    }
+
+    try {
+      adapter.toJsonTree(1);
+      fail();
+    } catch (JsonIOException e) {
+      assertEquals(exception, e.getCause());
+    }
+  }
+
   private static final TypeAdapter<String> adapter = new TypeAdapter<String>() {
     @Override public void write(JsonWriter out, String value) throws IOException {
       out.value(value);