search() is not type safe

[mirfilip]

New search() method has a contract to find enum by value and return corresponding key. Logically, this method should return boolean false when nothing found or a key, which is always a string (not integer), as internally key => value pairs are kept as associative array with string keys. Because search() uses array_search without strict param, it can find a wrong key:

class EnumFixture extends Enum
{
    const FOO = "bar";
    const PROBLEMATIC_NUMBER = 0;
    const PROBLEMATIC_NULL = null;
    const PROBLEMATIC_EMPTY_STRING = '';
    const PROBLEMATIC_BOOLEAN_FALSE = false;
}

EnumFixture::search(false); // PROBLEMATIC_NUMBER
EnumFixture::search(''); // PROBLEMATIC_NUMBER
EnumFixture::search(null); // PROBLEMATIC_NUMBER
EnumFixture::search('bar I do not exist'); // PROBLEMATIC_NUMBER

EnumFixture::search(array()); // PROBLEMATIC_NULL
EnumFixture::search(0); // FOO ***WTF***

This is the same kind of a problem as #9. The fix is to simply enable strict checking on search. Yes, you should not use such values, but the last search example shows problem even if you use proper values. I can take care of this if you want @mnapoli

[mirfilip]

Above there is a commit on my fork. It's based on changes from #9 which are not merged yet, so I will wait with this pull request until #9 is merged.

[mnapoli]

I've merged #12, feel free to create the other pull request.