[SPARK-16401][SQL] Data Source API: Enable Extending RelationProvider and CreatableRelationProvider without Extending SchemaRelationProvider

#### 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)`](https://github.com/gatorsmile/spark/blob/dd644f8117e889cebd6caca58702a7c7e3d88bef/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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