ci: try to get jruby green by pinning jar-dependencies (#3391)

**What problem is this PR intended to solve?**

Workaround recommended in jruby/jruby#7262

Gemfile

@@ -35,4 +35,5 @@ end
 # Then re-run `bundle install`.
 group :rdoc do
   gem "rdoc", "6.10.0"
+  gem "jar-dependencies", "0.4.1" if RUBY_PLATFORM == "java" # https://github.com/jruby/jruby/issues/7262
 end

ext/java/nokogiri/XmlXpathContext.java

@@ -154,7 +154,11 @@ public class XmlXpathContext extends RubyObject
   public IRubyObject
   register_ns(IRubyObject prefix, IRubyObject uri)
   {
-    nsContext.registerNamespace(prefix.asJavaString(), uri.asJavaString());
+    if (uri.isNil()) {
+      nsContext.deregisterNamespace(prefix.asJavaString());
+    } else {
+      nsContext.registerNamespace(prefix.asJavaString(), uri.asJavaString());
+    }
     return this;
   }
 
@@ -169,10 +173,23 @@ public class XmlXpathContext extends RubyObject
       variableResolver = NokogiriXPathVariableResolver.create();
       this.variableResolver = variableResolver;
     }
-    variableResolver.registerVariable(name.asJavaString(), value.asJavaString());
+    if (value.isNil()) {
+      variableResolver.deregisterVariable(name.asJavaString());
+    } else {
+      variableResolver.registerVariable(name.asJavaString(), value.asJavaString());
+    }
     return this;
   }
 
+  @JRubyMethod(name = "node=")
+  public IRubyObject
+  set_node(ThreadContext context, IRubyObject rb_node)
+  {
+    this.context = (XmlNode) rb_node;
+    return rb_node;
+  }
+
+
   private IRubyObject
   node_set(ThreadContext context, String expr, IRubyObject handler)
   {

ext/java/nokogiri/internals/NokogiriNamespaceContext.java

@@ -107,4 +107,10 @@ public final class NokogiriNamespaceContext implements NamespaceContext
     register.put(prefix, uri);
   }
 
+  public void
+  deregisterNamespace(String prefix)
+  {
+    if ("xmlns".equals(prefix)) { prefix = ""; }
+    register.remove(prefix);
+  }
 }

ext/java/nokogiri/internals/NokogiriXPathVariableResolver.java

@@ -34,4 +34,10 @@ public class NokogiriXPathVariableResolver implements XPathVariableResolver
   {
     variables.put(new QName(name), value);
   }
+  public void
+  deregisterVariable(String name)
+  {
+    variables.remove(new QName(name));
+  }
+
 }

test/xml/test_xpath_context.rb

@@ -16,19 +16,19 @@ module XML
         xc = XPathContext.new(doc)
 
         assert_raises(XPath::SyntaxError) do
-          xc.evaluate("//xmlns:child")
+          xc.evaluate("//foo:child")
         end
 
-        xc.register_namespaces({ "xmlns" => "http://nokogiri.org/default" })
+        xc.register_namespaces({ "foo" => "http://nokogiri.org/default" })
         assert_pattern do
-          xc.evaluate("//xmlns:child") => [
+          xc.evaluate("//foo:child") => [
             { name: "child", namespace: { href: "http://nokogiri.org/default" } }
           ]
         end
 
-        xc.register_namespaces({ "xmlns" => nil })
+        xc.register_namespaces({ "foo" => nil })
         assert_raises(XPath::SyntaxError) do
-          xc.evaluate("//xmlns:child")
+          xc.evaluate("//foo:child")
         end
       end