text_area should handle nil value option like text_field

Before:

    f.text_field :field, value: nil #=> <input value="">
    f.text_area :field, value: nil  #=> <textarea>value of field</textarea>

`text_field` uses `options.fetch` with a block to specify the default value, whereas `text_area` uses `||`. When given a value of `nil`, this makes `text_field` use the `nil` value because the key exists, whereas `text_area` will go for the default.

After:

    f.text_area :field, value: nil  #=> <textarea></textarea>

I modified `text_area` to also use a block. This makes both helpers behave the same way, which is IMO the expected behaviour when you explicitly pass `nil` as value.