[SPARK-9618] [SQL] Use the specified schema when reading Parquet files

The user specified schema is currently ignored when loading Parquet files.

One workaround is to use the `format` and `load` methods instead of `parquet`, e.g.:

```
val schema = ???

// schema is ignored
sqlContext.read.schema(schema).parquet("hdfs:///test")

// schema is retained
sqlContext.read.schema(schema).format("parquet").load("hdfs:///test")
```

The fix is simple, but I wonder if the `parquet` method should instead be written in a similar fashion to `orc`:

```
def parquet(path: String): DataFrame = format("parquet").load(path)
```

Author: Nathan Howell <nhowell@godaddy.com>

Closes #7947 from NathanHowell/SPARK-9618 and squashes the following commits:

d1ea62c [Nathan Howell] [SPARK-9618] [SQL] Use the specified schema when reading Parquet files

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

@@ -260,7 +260,7 @@ class DataFrameReader private[sql](sqlContext: SQLContext) extends Logging {
 
       sqlContext.baseRelationToDataFrame(
         new ParquetRelation(
-          globbedPaths.map(_.toString), None, None, extraOptions.toMap)(sqlContext))
+          globbedPaths.map(_.toString), userSpecifiedSchema, None, extraOptions.toMap)(sqlContext))
     }
   }