uix:TextInput convert to unicode only if string isn't already unicode

closes http://github.com/kivy/python-for-android/issues/114

Testing with the following code.

```
#-*- coding: utf-8 -*-
from kivy.app import App
from kivy.lang import Builder

root = Builder.load_string('''
TextInput:
    text: 'привіт світ'
''')

class TestApp(App):

    def build(self):
        #root.text = u'привіт світ'
        return root

if __name__ == '__main__':
    TestApp().run()
```