Gracefully handle json Encoding::UndefinedConversionError

[marcoadkins]

This addresses: #159 and #158

Things Done:

• Pass error logger through to formatter class
• Add exception handler for encoding errors
• Force utf 8 encoding and try again to format the event
• Unit tests

[codecov-commenter]

Codecov Report

Merging #165 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   92.68%   92.79%   +0.10%     
==========================================
  Files          64       64              
  Lines        1668     1735      +67     
==========================================
+ Hits         1546     1610      +64     
- Misses        122      125       +3     

Impacted Files	Coverage Δ
lib/logstash-logger/formatter.rb	96.87% <100.00%> (ø)
lib/logstash-logger/formatter/base.rb	100.00% <100.00%> (ø)
lib/logstash-logger/formatter/json.rb	100.00% <100.00%> (ø)
lib/logstash-logger/formatter/json_lines.rb	100.00% <100.00%> (ø)
lib/logstash-logger/logger.rb	83.60% <100.00%> (ø)
spec/formatter/base_spec.rb	100.00% <100.00%> (ø)
spec/formatter/json_lines_spec.rb	100.00% <100.00%> (ø)
spec/formatter/json_spec.rb	100.00% <100.00%> (ø)
lib/logstash-logger/device/kinesis.rb	88.88% <0.00%> (-5.56%)	⬇️
lib/logstash-logger/device/firehose.rb	88.88% <0.00%> (-5.56%)	⬇️
... and 1 more

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