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[ceonelson]

porting @AlexisLeon patch from n4kz/react-native-material-textfield#102

passes test:
> [email protected] jest C:\_oha\react-native-materialui-textfield
> jest "--silent"

 PASS  src\components\field\test.js
 PASS  src\components\label\test.js
 PASS  src\components\helper\test.js
 PASS  src\components\affix\test.js
 PASS  src\components\counter\test.js
 PASS  src\components\line\test.js

Test Suites: 6 passed, 6 total
Tests:       27 passed, 27 total
Snapshots:   26 passed, 26 total
Time:        2.097s
Done in 8.47s.

Using it successfully in my app :)