Change return type to RubyArray

The existing signature conflicts with one added to JRuby 9.2.9.
Specifically, the new signature in JRuby returns RubyArray, which
causes a compilation error on this line in Nokogiri because it
attempts to use a more general return type.

We would prefer to keep the specific return type in JRuby.

* If we patch JRuby, then 9.2.9 will never be able to compile any
  version of Nokogiri.
* If we patch Nokogiri, all versions of JRuby can compile current
  and future Nokogiri. Versions prior to 9.2.9 will be able to
  compile all existing releases of Nokogiri.

I do not believe the change in 9.2.9 breaks anything at runtime,
since the JVM does not care about this particular return type
mismatch unless someone actually returns a non-RubyArray object.

Fixes #1968

ext/java/nokogiri/XmlNodeSet.java

@@ -39,6 +39,7 @@
 import java.util.Arrays;
 
 import org.jruby.Ruby;
+import org.jruby.RubyArray;
 import org.jruby.RubyClass;
 import org.jruby.RubyFixnum;
 import org.jruby.RubyObject;
@@ -391,7 +392,7 @@ public IRubyObject subseq(ThreadContext context, int start, int length) {
     }
 
     @JRubyMethod(name = {"to_a", "to_ary"})
-    public IRubyObject to_a(ThreadContext context) {
+    public RubyArray to_a(ThreadContext context) {
         return context.runtime.newArrayNoCopy(nodes);
     }