Fixed NoMethodError when email object receives nil for text.

Upon instantiation of the Email object from Mandrill ```text``` was nil. This nil text was passed down to the ```clean_invalid_utf8_bytes``` method where the ```!text.valid_encoding?``` method was called on the nil object.

I am not familiar enough with the process to know if the text from an adapter should never be nil to begin with. This may reflect a larger underlying issue with how Mandrill emails are processed.