Fix error in Performance/Sum when method has no brackets

This fixes an issue where code like the following would cause an error
in the Performance/Sum cop:

  arr.inject :+

CHANGELOG.md

@@ -296,3 +296,4 @@
 [@dvandersluis]: https://github.com/dvandersluis
 [@ghiculescu]: https://github.com/ghiculescu
 [@mfbmina]: https://github.com/mfbmina
+[@mvz]: https://github.com/mvz

changelog/fix_fix_error_in_performancesum_when_method.md

@@ -0,0 +1 @@
+* [#264](https://github.com/rubocop/rubocop/pull/264): Fix error in Performance/Sum when method has no brackets. ([@mvz][])

lib/rubocop/cop/performance/sum.rb

@@ -156,7 +156,7 @@ def autocorrect_sum_map(corrector, sum, map, init)
         end
 
         def sum_method_range(node)
-          range_between(node.loc.selector.begin_pos, node.loc.end.end_pos)
+          range_between(node.loc.selector.begin_pos, node.loc.expression.end_pos)
         end
 
         def sum_map_range(map, sum)

spec/rubocop/cop/performance/sum_spec.rb

@@ -46,6 +46,17 @@
       RUBY
     end
 
+    it "registers an offense and corrects when using `array.#{method} 0, :+`" do
+      expect_offense(<<~RUBY, method: method)
+        array.#{method} 0, :+
+              ^{method}^^^^^^ Use `sum` instead of `#{method}(0, :+)`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        array.sum
+      RUBY
+    end
+
     it "registers an offense and corrects when using `array.#{method}(0.0, :+)`" do
       expect_offense(<<~RUBY, method: method)
         array.#{method}(0.0, :+)
@@ -88,6 +99,15 @@
 
         expect_no_corrections
       end
+
+      it 'does not autocorrect `:+` without brackets when initial value is not provided' do
+        expect_offense(<<~RUBY, method: method)
+          array.#{method} :+
+                ^{method}^^^ Use `sum` instead of `#{method}(:+)`, unless calling `#{method}(:+)` on an empty array.
+        RUBY
+
+        expect_no_corrections
+      end
     end
 
     context 'when `SafeAutoCorrect: false' do
@@ -114,6 +134,17 @@
           array.sum
         RUBY
       end
+
+      it 'autocorrects `:+` without brackets when initial value is not provided' do
+        expect_offense(<<~RUBY, method: method)
+          array.#{method} :+
+                ^{method}^^^ Use `sum` instead of `#{method}(:+)`, unless calling `#{method}(:+)` on an empty array.
+        RUBY
+
+        expect_correction(<<~RUBY)
+          array.sum
+        RUBY
+      end
     end
 
     it "registers an offense and corrects when using `array.#{method}(0, &:+)`" do