Handle Content-Types that are not :json, :xml, or :url_encoded_form

In c546a2b this was changed to mimic how the browser behaves in a real
situation but left out types that were registered.

When this was changed it didn't take `text/plain` or `text/html` content
types into account. This is a problem if you're manipulating the
`Content-Type` headers in your controller tests, and expect a certain
result.

The reason I changed this to use `to_sym` is because if the
`Content-Type` is not registered then the symbol will not exist. If it's
one of the special types we handle that specifically (:json, :xml, or
:url_encoded_form). If it's any registered type we handle it by setting
the `path_parameters` and then the `request_parameters`. If the `to_sym`
returns nil an error will be thrown.

If the controller test sets a `Content-Type` on the request that `Content-Type`
should remain in the header and pass along the filename.

For example:
If a test sets a content type on a post
```
@request.headers['CONTENT_TYPE'] = 'text/plain'
post :create, params: { name: 'foo.txt' }
```

Then `foo.txt` should be in the `request_parameters` and params related
to the path should be in the `path_parameters` and the `Content-Type`
header should match the one set in the `@request`. When c546a2b was
committed `text/plain` and `text/html` types were throwing a "Unknown
Content-Type" error which is misleading and incorrect.

Note: this does not affect how this is handled in the browser, just how
the controller tests handle setting `Content-Type`.

actionpack/lib/action_controller/test_case.rb

@@ -78,7 +78,9 @@ module ActionController
           # params parser middleware, and we should remove this roundtripping
           # when we switch to caling `call` on the controller
 
-          case content_mime_type.ref
+          case content_mime_type.to_sym
+          when nil
+            raise "Unknown Content-Type: #{content_type}"
           when :json
             data = ActiveSupport::JSON.encode(non_path_parameters)
             params = ActiveSupport::JSON.decode(data).with_indifferent_access
@@ -90,7 +92,8 @@ module ActionController
           when :url_encoded_form
             data = non_path_parameters.to_query
           else
-            raise "Unknown Content-Type: #{content_type}"
+            data = non_path_parameters.to_query
+            self.request_parameters = non_path_parameters
           end
         end
 

actionpack/test/controller/test_case_test.rb

@@ -627,6 +627,31 @@ XML
     assert_equal "application/json", parsed_env["CONTENT_TYPE"]
   end
 
+  def test_mutating_content_type_headers_for_plain_text_files_sets_the_header
+    @request.headers['Content-Type'] = 'text/plain'
+    post :render_body, params: { name: 'foo.txt' }
+
+    assert_equal 'text/plain', @request.headers['Content-type']
+    assert_equal 'foo.txt', @request.request_parameters[:name]
+    assert_equal 'render_body', @request.path_parameters[:action]
+  end
+
+  def test_mutating_content_type_headers_for_html_files_sets_the_header
+    @request.headers['Content-Type'] = 'text/html'
+    post :render_body, params: { name: 'foo.html' }
+
+    assert_equal 'text/html', @request.headers['Content-type']
+    assert_equal 'foo.html', @request.request_parameters[:name]
+    assert_equal 'render_body', @request.path_parameters[:action]
+  end
+
+  def test_mutating_content_type_headers_for_non_registered_mime_type_raises_an_error
+    assert_raises(RuntimeError) do
+      @request.headers['Content-Type'] = 'type/fake'
+      post :render_body, params: { name: 'foo.fake' }
+    end
+  end
+
   def test_id_converted_to_string
     get :test_params, params: {
       id: 20, foo: Object.new