When there is no value on an input, favor HTML's placeholder instead of katana's

[fmuttis]

For form extraction, the "value" attribute of an element is used.
When the value is not present, katana uses hardcoded placeholders. E.g. the value "katana".

This patch favors using the "placeholder" attribute of HTML tags if they are present, instead of directly using the hardcoded value.

So for the following form, katana will find "/?something=hello" instead of "/?something=katana":

<form method="GET">
<input type="text" name="something" value="" placeholder="hello">
</form>