[SPARK-3121] Wrong implementation of implicit bytesWritableConverter

val path = ... //path to seq file with BytesWritable as type of both key and value
val file = sc.sequenceFile[Array[Byte],Array[Byte]](path)
file.take(1)(0)._1

This prints incorrect content of byte array. Actual content starts with correct one and some "random" bytes and zeros are appended. BytesWritable has two methods:

getBytes() - return content of all internal array which is often longer then actual value stored. It usually contains the rest of previous longer values

copyBytes() - return just begining of internal array determined by internal length property

It looks like in implicit conversion between BytesWritable and Array[byte] getBytes is used instead of correct copyBytes.

@dbtsai

This test could use a better name.  I'd also add a comment, like `// Regression test for SPARK-3121` to help readers link this back to the JIRA.