using safestr in web.form.Dropdown._render_option

Convert to str (using safestr) before value comparison.

Old behavior (notice the second example):

```python
d = Dropdown(name='a', args=[1,2,3], value=1)
>>> d.render()
u'<select id="a" name="a">\n  <option selected="selected" value="1">1</option>\n  <option value="2">2</option>\n  <option value="3">3</option>\n</select>\n'
>>> 
>>> d = Dropdown(name='a', args=[1,2,3], value='1')
>>> d.render()
u'<select id="a" name="a">\n  <option value="1">1</option>\n  <option value="2">2</option>\n  <option value="3">3</option>\n</select>\n'
>>> 
```

New behavior (also notice the second example):

```python
>>> d = Dropdown(name='a', args=[1,2,3], value=1)
>>> d.render()
u'<select id="a" name="a">\n  <option selected="selected" value="1">1</option>\n  <option value="2">2</option>\n  <option value="3">3</option>\n</select>\n'
>>> 
>>> d = Dropdown(name='a', args=[1,2,3], value='1')
>>> d.render()
u'<select id="a" name="a">\n  <option selected="selected" value="1">1</option>\n  <option value="2">2</option>\n  <option value="3">3</option>\n</select>\n'
>>> 
```