Fix serial warnings

[yokolet]

What problem is this PR intended to solve?

This PR fixes all warnings of "warning: [serial] non-transient instance field of a serializable class declared with a non-serializable type", when Nokogiri is compiled on JRuby 9.4.

Fixing strategy is:

• The instance field type is from outside of Nokogiri: add transient declaration
• The instance field type is from Nokogiri but make it serializable requires many changes: add transient declaration
• The instance field type is from Nokogiri and make it serializable is simple: change the type to serializable

Have you included adequate test coverage?

No.

Does this change affect the behavior of either the C or the Java implementations?

It should not change any Java implementation behavior. The command bundle exec rake clean compile test result on JRuby 9.4 shows the changes don't break any.