String comparison using == in SignerProvider.java

In ```SignerProvider.java```, an exception is thrown using the condition ```this.signingKey == this.verifierKey```.  It appears the intent of this check is to compare the value of the strings, and so using ``equals()`` should be the correct way, instead of ==.  A test in ```SignerProviderTests.java``` seems to obey the same logic, and so I have updated the test to check for the condition that ```equals()``` fulfills.