Handle ConfigValues and ConfigSubstitution in HOCONConverter.to_hocon

[ChristopheDuong]

As also mentioned by Ivan@renardeinside on gitter, I encountered this bug and made a fix for it (plus a test):

Hi everyone.
I found something strange in HOCONConverter.convert(config,"hocon").
Probably i haven't enough understanding, but if you want to convert come config with unresolved substitution, you got strange result.
Suppose, you have file like this

parameter = ${substition.v1}

Then you parse this file and you won't resolve substitution:

config = ConfigFactory.parse_file("example.conf",resolve=False)

Then if you convert this config you got this result code:

print(HOCONConverter.convert(config,"hocon"))

output:

parameter = [ConfigValues: [ConfigSubstitution: substition.v1]]

But probably the result should be:

parameter = ${substition.v1}

[coveralls]

Coverage decreased (-0.4%) to 96.412% when pulling 40dc608 on ChristopheDuong:master into 0390324 on chimpler:master.

[darthbear]

Thank you @ChristopheDuong for your PR!

[darthbear]

Thanks @ChristopheDuong for your contribution. Your fix was pushed to pyhocon==0.3.48