Added Booleans.isBoolean to allow detecting whether a char sequence contains a boolean value. Added XContentParser.isBooleanValue to allow testing if current value is of boolean nature.

Boolean values:
boolean type values
numeric (0 == false)
one of the following strings: "true","false","on","off","yes","no","0","1"
Use XContentParser.isBooleanValue to detect all boolean values for the `_source` parameter for both _mget and _search

 Closes #3981