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

#### What changes were proposed in this pull request?
koertkuipers identified the PR https://github.com/apache/spark/pull/13727/ changed the behavior of `load` API. After the change, the `load` API does not add the value of `path` into the `options`. Thank you!

This PR is to add the option `path` back to `load()` API in `DataFrameReader`, if and only if users specify one and only one `path` in the `load` API. For example, users can see the `path` option after the following API call,
```Scala
spark.read
  .format("parquet")
  .load("/test")
```

#### How was this patch tested?
Added test cases.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #13933 from gatorsmile/optionPath.

sql/core/src/main/scala/org/apache/spark/sql/DataFrameReader.scala

@@ -129,7 +129,7 @@ class DataFrameReader private[sql](sparkSession: SparkSession) extends Logging {
    * @since 1.4.0
    */
   def load(path: String): DataFrame = {
-    load(Seq(path): _*) // force invocation of `load(...varargs...)`
+    option("path", path).load(Seq.empty: _*) // force invocation of `load(...varargs...)`
   }
 
   /**

sql/core/src/test/scala/org/apache/spark/sql/test/DataFrameReaderWriterSuite.scala

@@ -170,6 +170,34 @@ class DataFrameReaderWriterSuite extends QueryTest with SharedSQLContext with Be
     assert(LastOptions.saveMode === SaveMode.ErrorIfExists)
   }
 
+  test("test path option in load") {
+    spark.read
+      .format("org.apache.spark.sql.test")
+      .option("intOpt", 56)
+      .load("/test")
+
+    assert(LastOptions.parameters("intOpt") == "56")
+    assert(LastOptions.parameters("path") == "/test")
+
+    LastOptions.clear()
+    spark.read
+      .format("org.apache.spark.sql.test")
+      .option("intOpt", 55)
+      .load()
+
+    assert(LastOptions.parameters("intOpt") == "55")
+    assert(!LastOptions.parameters.contains("path"))
+
+    LastOptions.clear()
+    spark.read
+      .format("org.apache.spark.sql.test")
+      .option("intOpt", 54)
+      .load("/test", "/test1", "/test2")
+
+    assert(LastOptions.parameters("intOpt") == "54")
+    assert(!LastOptions.parameters.contains("path"))
+  }
+
   test("test different data types for options") {
     val df = spark.read
       .format("org.apache.spark.sql.test")