Fix how file_ and password_field_tag edit options

This commit fixes the behavior of `file_field_tag` and `password_field_tag`
when invoked with a hash of options.

These two helpers are different from all the other ones in that they modify the
options hash passed as a parameter, whereas all the other helpers duplicate it
before updating it.

The result is that *bad things* can happen if the user re-uses the same hash.
For instance, users who write the following code to display a file field
followed by a text field (both with the same class):

```rhtml
<% options = {class: 'important'} %>
<%= file_field_tag 'Upload', options %>
<%= text_field_tag 'Name', options %>
```

would instead see **two file fields!**

```html
<input class="important" id="Upload" name="Upload" type="file">
<input class="important" id="Name" name="Name" type="file" value="value">
```

This PR replaces `update` with `merge` in the code of the two helpers,
fixing the issue above.

The included test verifies the change, since it passes after this PR, but
fails before with the following error:

```
Expected: <input type="text" name="title" id="title" value="Hello!" class="important" />
Actual: <input type="password" name="title" id="title" value="Hello!" class="important" />
```

1 files changed, 7 insertions, 0 deletions

diff --git a/actionview/test/template/form_tag_helper_test.rb b/actionview/test/template/form_tag_helper_test.rb
index 771e3fefc3..2d89332841 100644
--- a/actionview/test/template/form_tag_helper_test.rb
+++ b/actionview/test/template/form_tag_helper_test.rb
@@ -170,6 +170,13 @@ class FormTagHelperTest < ActionView::TestCase
     assert_dom_equal expected, actual
   end
 
+  def test_multiple_field_tags_with_same_options
+    options = {class: 'important'}
+    assert_dom_equal %(<input name="title" type="file" id="title" class="important"/>), file_field_tag("title", options)
+    assert_dom_equal %(<input type="password" name="title" id="title" value="Hello!" class="important" />), password_field_tag("title", "Hello!", options)
+    assert_dom_equal %(<input type="text" name="title" id="title" value="Hello!" class="important" />), text_field_tag("title", "Hello!", options)
+  end
+
   def test_radio_button_tag
     actual = radio_button_tag "people", "david"
     expected = %(<input id="people_david" name="people" type="radio" value="david" />)