## What changes were proposed in this pull request?

- Fixed bug in Python API of DataStreamReader.  Because a single path was being converted to a array before calling Java DataStreamReader method (which takes a string only), it gave the following error.
```
File "/Users/tdas/Projects/Spark/spark/python/pyspark/sql/readwriter.py", line 947, in pyspark.sql.readwriter.DataStreamReader.json
Failed example:
    json_sdf = spark.readStream.json(os.path.join(tempfile.mkdtemp(), 'data'),                 schema = sdf_schema)
Exception raised:
    Traceback (most recent call last):
      File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/doctest.py", line 1253, in __run
        compileflags, 1) in test.globs
      File "<doctest pyspark.sql.readwriter.DataStreamReader.json[0]>", line 1, in <module>
        json_sdf = spark.readStream.json(os.path.join(tempfile.mkdtemp(), 'data'),                 schema = sdf_schema)
      File "/Users/tdas/Projects/Spark/spark/python/pyspark/sql/readwriter.py", line 963, in json
        return self._df(self._jreader.json(path))
      File "/Users/tdas/Projects/Spark/spark/python/lib/py4j-0.10.1-src.zip/py4j/java_gateway.py", line 933, in __call__
        answer, self.gateway_client, self.target_id, self.name)
      File "/Users/tdas/Projects/Spark/spark/python/pyspark/sql/utils.py", line 63, in deco
        return f(*a, **kw)
      File "/Users/tdas/Projects/Spark/spark/python/lib/py4j-0.10.1-src.zip/py4j/protocol.py", line 316, in get_return_value
        format(target_id, ".", name, value))
    Py4JError: An error occurred while calling o121.json. Trace:
    py4j.Py4JException: Method json([class java.util.ArrayList]) does not exist
    	at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:318)
    	at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:326)
    	at py4j.Gateway.invoke(Gateway.java:272)
    	at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:128)
    	at py4j.commands.CallCommand.execute(CallCommand.java:79)
    	at py4j.GatewayConnection.run(GatewayConnection.java:211)
    	at java.lang.Thread.run(Thread.java:744)
```

- Reduced code duplication between DataStreamReader and DataFrameWriter
- Added missing Python doctests

## How was this patch tested?
New tests

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #13703 from tdas/SPARK-15981.