add request method override for browsers

Browsers are not able to send put, delete or any other request from a
plain html form. This limits the possibilities with APIs so an override
was introduced in many frameworks in the form, that `_method` could be
defined in a post payload.
With this, zero also supports `_method` in the post payload to make it
possible to use all functions of the API with javascript through plain
html.

lib/zero/request.rb

@@ -68,7 +68,8 @@ module Zero
     # get the method of the request
     # @return [Symbol] the symbol representation of the method
     def method
-      @method ||= @env[CONST_REQUEST_METHOD].downcase.to_sym
+      return @method if @method
+      @method = extract_method
     end
     # is the method 'GET'?
     # @return [Boolean] true if this is a get request
@@ -103,5 +104,32 @@ module Zero
     # constant for the request method key
     # @api private
     CONST_REQUEST_METHOD = 'REQUEST_METHOD'
+    # regex to match for valid http methods
+    CONST_VALID_METHODS  = /\A(get|post|put|delete|head|link|unlink|patch)\Z/
+    # constant for post
+    CONST_POST           = 'post'
+    # constant for the method keyword
+    CONST_METHOD         = '_method'
+
+    # this function tries to figure out what method the request used
+    #
+    # The problem with extracting the request method is, that every client out
+    # there can speak all methods (get, post, put, whatever) but not browsers.
+    # When sending a form, they can only send get or post forms, but not put
+    # or delete, which makes them a pain to use. So instead an override was
+    # introduced in rails and other frameworks to support `_method` in the post
+    # payload.
+    # So this function does essentially the same, so please do not remove it
+    # until browsers learned how to do it.
+    # @return [Symbol] the extracted method
+    def extract_method
+      method = @env[CONST_REQUEST_METHOD].downcase
+      return method.to_sym unless method == CONST_POST
+      if params.payload.has_key?(CONST_METHOD)
+        method = params.payload[CONST_METHOD].downcase
+        return method.to_sym if CONST_VALID_METHODS.match(method)
+      end
+      CONST_POST.to_sym
+    end
   end
 end

spec/unit/zero/request/method_spec.rb

@@ -5,4 +5,47 @@ describe Zero::Request, '#method' do
   let(:env) { EnvGenerator.get('/foo') }
 
   its(:method) { should == :get }
+
+  context 'with post requests' do
+    context 'and _method defined' do
+      let(:env) do
+        EnvGenerator.post('/foo', {
+          :input => '_method=put',
+          'CONTENT_TYPE' => 'multipart/form-data'
+        })
+      end
+      it 'uses _method from the payload to change the method' do
+        expect(subject.method).to be(:put)
+      end
+    end
+
+    context 'and _method not defined' do
+      let(:env) do
+        EnvGenerator.post('/foo', {
+          :input => 'foo=bar',
+          'CONTENT_TYPE' => 'multipart/form-data'
+        })
+      end
+      its(:method) { should == :post }
+    end
+
+    context 'and _method has wrong content' do
+      let(:env) do
+        EnvGenerator.post('/foo', {
+          :input => '_method=foobar',
+          'CONTENT_TYPE' => 'multipart/form-data'
+        })
+      end
+      its(:method) { should == :post }
+    end
+
+    context 'and no payload' do
+      let(:env) do
+        EnvGenerator.post('/foo', {
+          'CONTENT_TYPE' => 'multipart/form-data'
+        })
+      end
+      its(:method) { should == :post }
+    end
+  end
 end