[SPARK-16236][SQL][FOLLOWUP] Add Path Option back to Load API in DataFrameReader

#### What changes were proposed in this pull request?
In Python API, we have the same issue. Thanks for identifying this issue, zsxwing ! Below is an example:
```Python
spark.read.format('json').load('python/test_support/sql/people.json')
```
#### How was this patch tested?
Existing test cases cover the changes by this PR

Author: gatorsmile <gatorsmile@gmail.com>

Closes #13965 from gatorsmile/optionPaths.

python/pyspark/sql/readwriter.py

@@ -143,7 +143,9 @@ class DataFrameReader(OptionUtils):
         if schema is not None:
             self.schema(schema)
         self.options(**options)
-        if path is not None:
+        if isinstance(path, basestring):
+            return self._df(self._jreader.load(path))
+        elif path is not None:
             if type(path) != list:
                 path = [path]
             return self._df(self._jreader.load(self._spark._sc._jvm.PythonUtils.toSeq(path)))