Fix: reading default files from .ini file

[deining]

When reading ini files, the function from_ini is called which takes three parameters. This function was called with only single parameter only, which caused the function to fail.

[Tieske]

@deining thx for the PR. Would you mind adding testcases showing the faulty behaviour and the fix?

[deining]

I added three test cases, as requested. Two of them are directly related to my PR.
The third test case #read_defaults_from_ini_file_group_with_cast is related to #64. It is currently disabled as there is no fix for this issue yet.

Side note: I came across another issue concerning whitespace handling in .ini files: trailing whitespace after the = do not work in groups other than [cli]. Using the config.ini below causes tests to fail:

[cli]
quiet = true
compress = bz2
[database]
host = localhost
port = 5432