[php] enhance type mapping

[wing328]

• map decimal to float
• revise logic in getting the type
• treat \DateTime, \SplFileObject as primitive types (not models) - except php-mezzio-ph, php-dt

Tested php client locally and the result is good:

................................................................. 65 / 73 ( 89%)
........                                                          73 / 73 (100%)

Time: 00:02.630, Memory: 12.00 MB

OK (73 tests, 220 assertions)

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cc @jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ackintosh (2017/09) ❤️, @ybelenko (2018/07), @renepardon (2018/12)

[wing328]

@Articus this PR results in some changes to php-dt, mezzio samples. Can you please review to see if you're ok with these changes?

[Articus]

Treating \DateTime and \SplFileObject as primitive types breaks code generated by both php-mezzio-ph and php-dt :( In terms of PHP language these types are not scalar and generator templates were written assuming that isPrimitiveType and isDateTime are mutually exclusive flags. So now in generated code serializator is given conflicting instructions how to handle DTO fields with these types.

I can investigate this issue further during weekend to check what else should be adjusted.

[wing328]

@Articus thanks for the review. I'll try to workaround it.

[wing328]

@Articus worked around via 6ed7568. Please review the latest again when you've time. Thanks.

[wing328]

Merged as I've other enhancements for the php client generator.