Work around respond_to? behavior in JRuby 1.6.13

In JRuby 1.6.13 calling respond_to? with :method_missing returns false
even if method_missing is defined. This commit replaces the checks that
relied on respond_to? returning true in this case.
Use :: instead of const_get
Allow and and or for control flow
Remove unused method
Bring IBaseInfo#deprecated? under test
Simplify method
Make GValue#set_value return nil

This matches the return value of the specialized #get_<type> methods.
Make GObject::Value#value= an alias of #set_value
Remove unused methods from UnintrospectableTypeInfo
Remove unused methods from FieldBuilder
Use GObject::Object#is_floating directly
Remove unused object ref methods
Bring ITypeInfo#name under test