Ensure that the string is truncated only when necessary. (Issue #54)

Thanks to @rafaelpetry for writing the issue #54.

The string was being truncated even when it was shorter than the length param (when using omission). This was happening because in order to make sure that the result (truncated string with the omission) wasn't larger than the desired length, the string was being truncated only to the desired length minus the omission length. 

Now, it checks first if the string is shorter or equal than the desired length, if it is, it returns the string without processing. This way we don't have to mess with the truncation pipeline, and the string doesn't go through the process if it's not necessary.

Please let me know if you think that I should change or do something different to solve the issue!