#69 DatasetProductionTimesKey does not produce distinct keys

[Tom Porter]

In
de.laures.cewolf.taglib.util.DatasetProductionTimesKey's
constructor KeyGenerator.generateKey is called with
the `this` reference. Based on the definition of the
class, `this` contains no useful, distinct members for
creation of a unique key; therefore each time the
constructor is called the same key value is produced not
allowing datasets to be cached properly.

[Tom Porter]

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Altered the call to generateKey() to use the Map params, as
it will be unique per producer and its parameters, instead of
the `this` reference. Change available in
file ../de/laures/cewolf/taglib/util/DatasetProductionTimesKey.j
ava, version 1.4.