fix ignore newline by after textarea's opening tag

The textarea tag is missing first newline. I want to save the input as content.

From the [HTML 4.01 specification appendices:](http://www.w3.org/TR/html401/appendix/notes.html#notes-line-breaks)

> SGML (see [ISO8879], section 7.6.1) specifies that a line break immediately following a start tag must be ignored, as must a line break immediately before an end tag. This applies to all HTML elements without exception.
> The following two HTML examples must be rendered identically:

So insert newline after the textarea opening tag.

for example

~~~
text_area_tag :test, value: "\n\nfoo\nbar"
~~~

output html is

~~~
<textarea name="test">\n\nfoo\nbar</textarea>
~~~

But what is sent to the server is the only "\nfoo\nbar".

I want to solve this problem.