## What changes were proposed in this pull request?
Currently column names of headers in CSV files are not checked against provided schema of CSV data. It could cause errors like showed in the [SPARK-23786](https://issues.apache.org/jira/browse/SPARK-23786) and https://github.com/apache/spark/pull/20894#issuecomment-375957777. I introduced new CSV option - `enforceSchema`. If it is enabled (by default `true`), Spark forcibly applies provided or inferred schema to CSV files. In that case, CSV headers are ignored and not checked against the schema. If `enforceSchema` is set to `false`, additional checks can be performed. For example, if column in CSV header and in the schema have different ordering, the following exception is thrown:
```
java.lang.IllegalArgumentException: CSV file header does not contain the expected fields
Header: depth, temperature
Schema: temperature, depth
CSV file: marina.csv
```
## How was this patch tested?
The changes were tested by existing tests of CSVSuite and by 2 new tests.
Author: Maxim Gekk <maxim.gekk@databricks.com>
Author: Maxim Gekk <max.gekk@gmail.com>
Closes#20894 from MaxGekk/check-column-names.
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