[Minor] [SQL] Renames stringRddToDataFrame to stringRddToDataFrameHolder for consistency

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Author: Cheng Lian <lian@databricks.com>

Closes #4613 from liancheng/df-implicit-rename and squashes the following commits:

db8bdd3 [Cheng Lian] Renames stringRddToDataFrame to stringRddToDataFrameHolder for consistency

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

@@ -235,7 +235,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
     }
 
     /** Creates a single column DataFrame from an RDD[String]. */
-    implicit def stringRddToDataFrame(data: RDD[String]): DataFrameHolder = {
+    implicit def stringRddToDataFrameHolder(data: RDD[String]): DataFrameHolder = {
       val dataType = StringType
       val rows = data.mapPartitions { iter =>
         val row = new SpecificMutableRow(dataType :: Nil)