[SPARK-15308][SQL] RowEncoder should preserve nested column name.

## What changes were proposed in this pull request?

The following code generates wrong schema:

```
val schema = new StructType().add(
  "struct",
  new StructType()
    .add("i", IntegerType, nullable = false)
    .add(
      "s",
      new StructType().add("int", IntegerType, nullable = false),
      nullable = false),
  nullable = false)
val ds = sqlContext.range(10).map(l => Row(l, Row(l)))(RowEncoder(schema))
ds.printSchema()
```

This should print as follows:

```
root
 |-- struct: struct (nullable = false)
 |    |-- i: integer (nullable = false)
 |    |-- s: struct (nullable = false)
 |    |    |-- int: integer (nullable = false)
```

but the result is:

```
root
 |-- struct: struct (nullable = false)
 |    |-- col1: integer (nullable = false)
 |    |-- col2: struct (nullable = false)
 |    |    |-- col1: integer (nullable = false)
```

This PR fixes `RowEncoder` to preserve nested column name.

## How was this patch tested?

Existing tests and I added a test to check if `RowEncoder` preserves nested column name.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #13090 from ueshin/issues/SPARK-15308.