Map should fill null values only when set WriteMapNullValue

.gitignore

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+/target
+/.project
+/.settings
+/.classpath
+/.idea
+/.DS_Store
+*.iml
+
+/bin/

.travis.yml

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+language: java
+jdk:
+  - oraclejdk8
+before_install:
+  - pip install --user codecov
+after_success:
+  - codecov
+branches:
+  except:
+    - appveyor

README.md

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+# fastjson
+
+[![Build Status](https://travis-ci.org/alibaba/fastjson.svg?branch=master)](https://travis-ci.org/alibaba/fastjson)
+[![Codecov](https://codecov.io/gh/alibaba/fastjson/branch/master/graph/badge.svg)](https://codecov.io/gh/alibaba/fastjson/branch/master)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.alibaba/fastjson/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.alibaba/fastjson/)
+[![GitHub release](https://img.shields.io/github/release/alibaba/fastjson.svg)](https://github.com/alibaba/fastjson/releases)
+[![License](https://img.shields.io/badge/license-Apache%202-4EB1BA.svg)](https://www.apache.org/licenses/LICENSE-2.0.html)
+
+Fastjson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Fastjson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of.
+
+### Fastjson Goals
+ * Provide best performance in server side and android client
+ * Provide simple toJSONString() and parseObject() methods to convert Java objects to JSON and vice-versa
+ * Allow pre-existing unmodifiable objects to be converted to and from JSON
+ * Extensive support of Java Generics
+ * Allow custom representations for objects
+ * Support arbitrarily complex objects (with deep inheritance hierarchies and extensive use of generic types)
+
+![fastjson](logo.jpg "fastjson")
+
+## Documentation
+
+- [Documentation Home](https://github.com/alibaba/fastjson/wiki)
+- [Frequently Asked Questions](https://github.com/alibaba/fastjson/wiki/%E5%B8%B8%E8%A7%81%E9%97%AE%E9%A2%98)
+
+## Benchmark
+
+https://github.com/eishay/jvm-serializers/wiki
+
+## Download
+
+- [maven][1]
+- [the latest JAR][2]  
+
+[1]: http://repo1.maven.org/maven2/com/alibaba/fastjson/  
+[2]: https://search.maven.org/remote_content?g=com.alibaba&a=fastjson&v=LATEST
+
+## Maven
+
+```xml
+<dependency>
+    <groupId>com.alibaba</groupId>
+    <artifactId>fastjson</artifactId>
+    <version>1.2.46</version>
+</dependency>
+```
+
+```xml
+<dependency>
+    <groupId>com.alibaba</groupId>
+    <artifactId>fastjson</artifactId>
+    <version>1.1.68.android</version>
+</dependency>
+```
+
+## Gradle via JCenter
+
+``` groovy
+compile 'com.alibaba:fastjson:1.2.46'
+```
+
+``` groovy
+compile 'com.alibaba:fastjson:1.1.68.android'
+```
+
+Please see this [Wiki Download Page][Wiki] for more repository infos.
+
+[Wiki]: https://github.com/alibaba/fastjson/wiki#download
+
+### *License*
+
+Fastjson is released under the [Apache 2.0 license](license.txt).
+
+```
+Copyright 1999-2017 Alibaba Group Holding Ltd.
+
+Licensed 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 following link.
+
+     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.
+```

license.txt

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+Copyright 1999-2016 Alibaba Group Holding Ltd.
+
+Licensed 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.