Move to_java into java_producer to make push_single signature match MRI

lib/hermann/producer.rb

@@ -63,8 +63,7 @@ module Hermann
       end
 
       if Hermann.jruby?
-        key = opts.has_key?(:partition_key) ? opts[:partition_key].to_java : nil
-        result = @internal.push_single(value, topic, key, nil)
+        result = @internal.push_single(value, topic, opts[:partition_key], nil)
         unless result.nil?
           @children << result
         end

lib/hermann/provider/java_producer.rb

@@ -44,6 +44,7 @@ module Hermann
       #   data to the kafka broker.  Upon execution the Promise's status
       #   will be set
       def push_single(msg, topic, key, _)
+        key = key && key.to_java
         Concurrent::Promise.execute {
           data = ProducerUtil::KeyedMessage.new(topic, nil, key, msg.to_java_bytes)
           begin

spec/producer_spec.rb

@@ -68,13 +68,7 @@ describe Hermann::Producer do
       end
     end
 
-    context 'with explicit partition key', :platform => :java do
-      it 'uses the partition key' do
-        expect(producer.internal).to receive(:push_single).with(msg, topic, partition_key.to_java, anything)
-        producer.push(msg, :partition_key => partition_key)
-      end
-    end
-    context 'with explicit partition key', :platform => :mri do
+    context 'with explicit partition key' do
       it 'uses the partition key' do
         expect(producer.internal).to receive(:push_single).with(msg, topic, partition_key, anything)
         producer.push(msg, :partition_key => partition_key)