Fix tests warning and notices

[davidmpaz]

Hi,

thanks for maintaining this project. Could we include this on the main-stream, these are some details I noticed when running the tests for the project when creating #536

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Add returns types to HashMap interface.
Correct namespace on IssuerTest.
Avoid using warning silence operator.

[Halleck45]

Hi,

I cannot accept this PR : it breaks retro-compatibility.

It's sometime hard, but it's probably possible to keep the compatibility with all PHP versions, even in this case.

Edit : I've just merged #538 to fix the depreciations on CI

[davidmpaz]

Hi,

sure totally understandable. Let me check on this again. Maybe there is a sweet spot for reducing some warning while keeping fully backward compatible

Thanks
David