validate the text encoding and replace the text with a String#inspect

value if it is not valid.

This is the first of what will likely be an endless road of trying to
best solve the "My data is not in UTF-8 and I have no idea what to do
next!" problem that continues to plague logstash users.

The new behavior is that invalidly-encoded text will be replaced with a
String#inspect value which should help identify the original text
through an escape sequence and feels better than using ruby's encoding
replace-character support (which loses the data during conversion)

LOGSTASH-1443