Accept an array of properties in get/getLiteral/getResource methods

[osma]

This PR changes the implementation of EasyRdf\Graph::get to accept an array of properties. Previously the implementation was inconsistent with the phpdoc declarations: the phpdocs for getLiteral and getResource methods in both Graph and Resource classes state that it is possible to pass an array of properties, but it didn't actually work (see #325). Also the phpdoc description for Graph::get hints that it is possible to pass an array.

The PR straightens out this wrinkle so that it is actually possible to pass an array to all the above mentioned methods. The actual fix was simple - only two new lines of code - but then I had to change the phpdoc a little to reflect that, and also the exception message that complains about bad values now makes it clear that an array is accepted, too.

Finally I had to do small changes to existing unit tests that check for the exception message, plus I added two more to verify that passing an array of properties actually works as intended.

Fixes #325, ping @k00ni

[k00ni]

👍 for merging it

[k00ni]

@njh and @zozlak, any objections? @indeyets approved it.

[zozlak]

I'm pretty sure the fix won't work for fully-qualified URIs (e.g. $resource->get(['https://foo/test', 'https://:foo/bar']);.

But it's a part of a more general issue of accepting a property-like parameter of a mixed type and the way it's handled internally in the EasyRdf.

[zozlak]

By the way, it will probably work for $resource->get(['<https://foo/test>', '<https://:foo/bar>']); but this should be stated in the documentation.

And one more scenario to be tested is $resource->get([$graph->resource('https://foo/test'), $graph->resource('https://foo/test')]);

[k00ni]

👍

Any objections @njh?

[njh]

Looks good to me 👍