ARROW-1340: [Java] Fix NullableMapVector field metadata

Author: Emilio Lahr-Vivaz <elahrvivaz@ccri.com>

Closes #953 from elahrvivaz/ARROW-1340 and squashes the following commits:

a307779 [Emilio Lahr-Vivaz] ARROW-1340: [Java] Fix NullableMapVector field metadata

Author: elahrvivaz

java/vector/src/main/java/org/apache/arrow/vector/complex/NullableMapVector.java

@@ -86,7 +86,8 @@ public NullableMapVector(String name, BufferAllocator allocator, FieldType field
   @Override
   public Field getField() {
     Field f = super.getField();
-    return new Field(f.getName(), true, f.getType(), f.getChildren());
+    FieldType type = new FieldType(true, f.getType(), f.getFieldType().getDictionary(), f.getFieldType().getMetadata());
+    return new Field(f.getName(), type, f.getChildren());
   }
 
   @Override

java/vector/src/test/java/org/apache/arrow/vector/TestMapVector.java

@@ -0,0 +1,57 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.arrow.vector;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.vector.complex.NullableMapVector;
+import org.apache.arrow.vector.types.pojo.ArrowType.Struct;
+import org.apache.arrow.vector.types.pojo.FieldType;
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+
+public class TestMapVector {
+
+  private BufferAllocator allocator;
+
+  @Before
+  public void init() {
+    allocator = new DirtyRootAllocator(Long.MAX_VALUE, (byte) 100);
+  }
+
+  @After
+  public void terminate() throws Exception {
+    allocator.close();
+  }
+
+  @Test
+  public void testFieldMetadata() throws Exception {
+    Map<String, String> metadata = new HashMap<>();
+    metadata.put("k1", "v1");
+    FieldType type = new FieldType(true, Struct.INSTANCE, null, metadata);
+    try (NullableMapVector vector = new NullableMapVector("map", allocator, type, null)) {
+      Assert.assertEquals(vector.getField().getMetadata(), type.getMetadata());
+    }
+  }
+}