Use LinkedHashMap for deterministic iteration order

The test opennlp.tools.ml.naivebayes.NaiveBayesSerializedCorrectnessTest#testPlainTextModel can fail due to a different iteration order of HashMap. The error message is as follows:

{{[ERROR] testPlainTextModel(opennlp.tools.ml.naivebayes.NaiveBayesSerializedCorrectnessTest) Time elapsed: 0.01 s <<< FAILURE!
org.junit.ComparisonFailure:
expected:<...ports
...}}

The fix is to change HaskMap to LinkedHashMap so that the iteration order remains stable and the failure will not occur any more. In this way, the test will be more stable.