[SPARK-1998] SparkFlumeEvent with body bigger than 1020 bytes are not re...

flume event sent to Spark will fail if the body is too large and numHeaders is greater than zero

Author: joyyoj <sunshch@gmail.com>

Closes #951 from joyyoj/master and squashes the following commits:

f4660c5 [joyyoj] [SPARK-1998] SparkFlumeEvent with body bigger than 1020 bytes are not read properly