#### What changes were proposed in this pull request?
When users try to implement a data source API with extending only `RelationProvider` and `CreatableRelationProvider`, they will hit an error when resolving the relation.
```Scala
spark.read
.format("org.apache.spark.sql.test.DefaultSourceWithoutUserSpecifiedSchema")
.load()
.write.
format("org.apache.spark.sql.test.DefaultSourceWithoutUserSpecifiedSchema")
.save()
```
The error they hit is like
```
org.apache.spark.sql.test.DefaultSourceWithoutUserSpecifiedSchema does not allow user-specified schemas.;
org.apache.spark.sql.AnalysisException: org.apache.spark.sql.test.DefaultSourceWithoutUserSpecifiedSchema does not allow user-specified schemas.;
at org.apache.spark.sql.execution.datasources.DataSource.resolveRelation(DataSource.scala:319)
at org.apache.spark.sql.execution.datasources.DataSource.write(DataSource.scala:494)
at org.apache.spark.sql.DataFrameWriter.save(DataFrameWriter.scala:211)
```
Actually, the bug fix is simple. [`DataSource.createRelation(sparkSession.sqlContext, mode, options, data)`](dd644f8117/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSource.scala (L429)) already returns a BaseRelation. We should not assign schema to `userSpecifiedSchema`. That schema assignment only makes sense for the data sources that extend `FileFormat`.
#### How was this patch tested?
Added a test case.
Author: gatorsmile <gatorsmile@gmail.com>
Closes#14075 from gatorsmile/dataSource.
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