Data mapper reader management

Hi Guys,

This PR contains two cosmetic changes in `junitparams.mappers.DataMapper` implementations:

1) Removed duplication of logic reading files with `BufferedReader` in the following classes:
- `junitparams.mappers.CsvWithHeaderMapper`
- `junitparams.mappers.IdentityMapper`

2) Removed line closing Reader in Data Mapper implementations.
The reader lifecycle is externally managed by `junitparams.internal.parameters.ParametersFromFile` as it is mentioned in javadoc for `junitparams.mappers.DataMapper#map` - "...The reader is closed in the framework, so just read it :)..."