sanitize_text_field() assumes the field is a string

[johnbillion]

The sanitize_text_field() sanitisation function is used to sanitize text input, but the function actually assumes the field is a string. If an array is passed in, for example, then it'll raise PHP errors.

This function should gracefully handle not string data, probably by returning an empty string.

sanitize_textarea_field has the same issue. Should both functions check whether the input is a string independently, or should _sanitize_text_fields do it?

patch with unit test

This patch contain a check with is_string with unit test for a string that is not a string.