FCPPluginMessage: Implement test to check encoding/decoding into the
on-network format

- See its class-level JavaDoc for how it works.
- It was indicated to write this test for the following reasons:
* Class FCPPluginClient recently received code for dumping all
sent/received FCPPluginMessage to the Logger
* The dumping code was easiest to implement by dumping the high level
FCPPluginMessage objects. In other words, it does NOT dump the actual
data which hits the network.
* Thus, the logs cannot be used to check whether what hits the network
is correct
* Additionally, the code in Web of Trust which I plan to use to test
this fred branch also cannot dump raw network traffic anymore, and it
would be the wrong place to implement that there.
* So we should instead at least test raw network traffic with unit
tests.
FCPPluginServerMessage: Fix encoding of messages with empty
FCPPluginMessage.params

- This is needed for being able to send mere "ACK"-reply
FCPPluginMessages: They will only contain an identifier and
Success=true. (Which is needed for FCPPluginClient.sendSynchronous()).

- Symptom would be caught by FCPPluginMessageEncodeDecodeTest:
java.lang.IllegalArgumentException: Empty
	at freenet.support.SimpleFieldSet.put(SimpleFieldSet.java:824)
	at freenet.node.fcp.FCPPluginServerMessage
		.getFieldSet(FCPPluginServerMessage.java:158)
	at freenet.node.fcp.FCPPluginMessageEncodeDecodeTest
		.testEncodeDecode(FCPPluginMessageEncodeDecodeTest.java:113)
	at freenet.node.fcp.FCPPluginMessageEncodeDecodeTest
		.testEncodeDecode(FCPPluginMessageEncodeDecodeTest.java:88)