GROOVY-6554: Improved serialization performance of JsonOutput
GROOVY-6633: In each JsonOutput.toJson method added check for null
Merge pull request #362 from bura/fix-null-to-json

GROOVY-6633: In each JsonOutput.toJson method added check for null
Merge branch 'impr-json-output' of github.com:bura/groovy-core into bura-impr-json-output

Conflicts:
	subprojects/groovy-json/src/main/groovy/groovy/json/JsonOutput.groovy
	subprojects/groovy-json/src/test/groovy/groovy/json/JsonOutputTest.groovy