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" />
```

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -263,7 +263,7 @@ module ActionView
       #   file_field_tag 'file', accept: 'text/html', class: 'upload', value: 'index.html'
       #   # => <input accept="text/html" class="upload" id="file" name="file" type="file" value="index.html" />
       def file_field_tag(name, options = {})
-        text_field_tag(name, nil, options.update("type" => "file"))
+        text_field_tag(name, nil, options.merge(type: :file))
       end
 
       # Creates a password field, a masked text field that will hide the users input behind a mask character.
@@ -296,7 +296,7 @@ module ActionView
       #   password_field_tag 'pin', '1234', maxlength: 4, size: 6, class: "pin_input"
       #   # => <input class="pin_input" id="pin" maxlength="4" name="pin" size="6" type="password" value="1234" />
       def password_field_tag(name = "password", value = nil, options = {})
-        text_field_tag(name, value, options.update("type" => "password"))
+        text_field_tag(name, value, options.merge(type: :password))
       end
 
       # Creates a text input area; use a textarea for longer text inputs such as blog posts or descriptions.

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" />)