Allow empty string as source language for auto-detection

[boryn]

This pull request reverts possibility of automatic source language detection.

Normally, when we do not pass 'source' in $options array to Google, the translation engine tries to detect it automatically (without an additional API call).

Now, it's possible to use empty string as the source language either explicitly at translate() / translateBatch() methods or generally in config at 'default_source_translation'.