## What changes were proposed in this pull request?
For a class with field name of special characters, e.g.:
```scala
case class MyType(`field.1`: String, `field 2`: String)
```
Although we can manipulate DataFrame/Dataset, the field names are encoded:
```scala
scala> val df = Seq(MyType("a", "b"), MyType("c", "d")).toDF
df: org.apache.spark.sql.DataFrame = [field$u002E1: string, field$u00202: string]
scala> df.as[MyType].collect
res7: Array[MyType] = Array(MyType(a,b), MyType(c,d))
```
It causes resolving problem when we try to convert the data with non-encoded field names:
```scala
spark.read.json(path).as[MyType]
...
[info] org.apache.spark.sql.AnalysisException: cannot resolve '`field$u002E1`' given input columns: [field 2, fie
ld.1];
[info] at org.apache.spark.sql.catalyst.analysis.package$AnalysisErrorAt.failAnalysis(package.scala:42)
...
```
We should use decoded field name in Dataset schema.
## How was this patch tested?
Added tests.
Author: Liang-Chi Hsieh <viirya@gmail.com>
Closes#19664 from viirya/SPARK-22442.
40a8aefaf3