Move options check into a separate method
Bufgix: do not modify the source string (causes boost to be applied multiple times, e.g. field_name^2^2^2 adding it again and again with each new call)
Fix typo
Bugfix: do not mix fulltext and joined fields when adding from the fulltext DSL
Move common methods to the abstract class
Bugfix: update Field#eql? to compare fields of different classes properly