make DateTimeZoneSerializer set the type to "string" to improve schema support (currently the type will be any)

Author: jwijgerd

src/main/java/com/fasterxml/jackson/datatype/joda/ser/DateTimeZoneSerializer.java

@@ -2,6 +2,9 @@
 
 import java.io.IOException;
 
+import com.fasterxml.jackson.databind.JavaType;
+import com.fasterxml.jackson.databind.JsonMappingException;
+import com.fasterxml.jackson.databind.jsonFormatVisitors.JsonFormatVisitorWrapper;
 import org.joda.time.DateTimeZone;
 
 import com.fasterxml.jackson.core.JsonGenerator;
@@ -32,4 +35,10 @@ public void serializeWithType(DateTimeZone value, JsonGenerator g,
         serialize(value, g, provider);
         typeSer.writeTypeSuffix(g, typeIdDef);
     }
+
+    @Override
+    public void acceptJsonFormatVisitor(JsonFormatVisitorWrapper visitor, JavaType typeHint)
+            throws JsonMappingException {
+        visitor.expectStringFormat(typeHint);
+    }
 }