Fix indent and add test case

[chenhan1218]

Fixes #415

Changes proposed in this pull request:

• Fix indent parameter parsing by correct the format.
• Add test case for encode function with indent argument.

[chenhan1218]

$ pip install .
$ python -c 'import ujson; print(ujson.encode({"a":1}, indent=8))'  
{
        "a": 1
}

[hugovk]

Thank you!