Follow up of #66

This aligns the field with the latest changes in
symphonycms/symphony-2#2100

It now implements `prepareTextValue` but delegates reads to the default
`prepareReadableValue` implementation that will properly format
our implementation of textValue.
Check if $data is array before checking isset

I do not know why, but on my setup, when $data is a string, isset would
return true. Also checking if $data is an array fixes it and should not
cause damage since the only thing we do with tha array is filter it.
Merge pull request #71 from DeuxHuitHuit/php-is-set-issue

Check if $data is array before checking isset. RE: #70
Fix filtering with contains: in the backend. Closes symphonycms/symphon-2#2074
Merge pull request #69 from DeuxHuitHuit/prepare-text

Follow up of #66