[FLINK-2220] Join on Pojo without hashCode() silently fails

[gallenvara]

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Add a check to verify the POJO has overridden the hashCode() and equals() where it used as a key for operations(join,coGroup,etc).

[fhueske]

I added a comment to FLINK-2220. I think the original analysis of the problem was not correct. It is not necessary to check for POJOs whether they override equals() and hashcode(). Details in FLINK-2220

[fhueske]

Any update for this PR, @gallenvara?

[gallenvara]

@fhueske i can't reproduce the issue in another computer. And I'm not sure if it's because I forgot the default constructor (no parameter) that led to the problem existing. I sent email to the reporter and not responsed. I will close the PR. Thanks.