[FLINK-2437] Fix default constructor detection in TypeExtractor.analyzePojo

[ggevay]

• handle the case of a non-public default constructor
• added a missing "return null"
• removed some duplicating of the word "class" when printing class
  names, because class.toString also prepends it to the class name
• fixed a typo in the documentation describing POJOs

[ggevay]

I removed the added "return null", I just realized that it is not needed.

[gyfora]

Could you please add a test case that verifies the correct behaviour?

[StephanEwen]

Why are you changing all messages in this pull request? They were not wrong before...

[mxm]

Stephan is right. The changed messages just add noise to the pull request which makes it harder to review.

[ggevay]

They print things like
Class class malom.GameState must have a default constructor to be used as a POJO.
If you think that the double "class" should be left the way it is, then I'm sorry for changing them. I have now removed the changes from the PR.

I have also added a test.

[StephanEwen]

Removing the double "class" is actually not bad. Was hard to get this from the diff alone.

BTW: Simpler way may be to change the aClass.toString() call to aClass.getCanonicalName().

[ggevay]

OK, I re-added the changes to remove the double "class". (Note, that the 3rd bullet in the opening comment refers to this doubling.)

[mxm]

if(defaultConstructor != null && Modifier.isPublic(defaultConstructor.getModifiers()) seems to be more readable to me but your approach is fine too.

[ggevay]

You are right, I have changed it.

[mxm]

Your changes look good to merge.

[mxm]

Thanks for your contribution!