Fix issue where the textfield would be cleared when losing focus iOS 10

[jvanzummeren]

Solves #4

Bug was caused by overwriting text and attributedText and setting the getter value to nil

[luximetr]

I am not sure about this. Probably i had problems connected with that. I need time to remember and test this.

[jvanzummeren]

I understand, it's a sort of fundamental change. A few additional comments to make you feel more comfortable with the fix.

1. I changed the implementation to make use of the 'content' and 'attributedContent' instead of calling super to make it more in line with the existing implementation.
2. I tested it on iOS 10, 11 and 12
3. Only iOS 10 is calling the 'original' attributedText property when 'resigning FirstResponder', although iOS 10, 11 and 12 call it when backspacing last character
4. I tested it with both starting with nothing, programatically setting .content and/or .attributedContent

:)