Fix single-argument lambda regression in exec_with_object

PR #389 changed arity detection to use block.arity.zero? which broke
single-argument lambdas in expose blocks and if: conditions, causing
ArgumentError (given 2, expected 1). Restore parameter count check
for regular blocks while keeping symbol-to-proc detection intact.

Fixes #398

Author: numbata

lib/grape_entity/entity.rb

@@ -525,13 +525,10 @@ def serializable_hash(runtime_options = {})
     end
 
     def exec_with_object(options, &block)
-      arity = if symbol_to_proc_wrapper?(block)
-                ensure_block_arity!(block)
-              else
-                block.arity
-              end
-
-      if arity.zero?
+      if symbol_to_proc_wrapper?(block)
+        ensure_block_arity!(block)
+        instance_exec(object, &block)
+      elsif block.parameters.one?
         instance_exec(object, &block)
       else
         instance_exec(object, options, &block)

spec/grape_entity/entity_spec.rb

@@ -476,6 +476,30 @@ def raises_argument_error
               end.to raise_error ArgumentError, match(/method is not defined in the object/)
             end
           end
+
+          context 'with single-argument lambda' do
+            it 'passes only the object without raising ArgumentError' do
+              subject.expose :that_method_without_args do |object|
+                object.method_without_args
+              end
+
+              object = SomeObject.new
+              value = subject.represent(object).value_for(:that_method_without_args)
+              expect(value).to eq('result')
+            end
+          end
+
+          context 'with two-argument lambda' do
+            it 'passes the object and options without raising ArgumentError' do
+              subject.expose :that_method_without_args do |object, _options|
+                object.method_without_args
+              end
+
+              object = SomeObject.new
+              value = subject.represent(object).value_for(:that_method_without_args)
+              expect(value).to eq('result')
+            end
+          end
         end
 
         context 'with no parameters passed to the block' do
@@ -521,6 +545,32 @@ def raises_argument_error
             expect(subject.represent({}).value_for(:awesome)).to eq(condition_met: 'value')
           end
 
+          it 'works with single-argument if condition lambdas' do
+            subject.expose :awesome do
+              subject.expose(:condition_met, if: ->(_) { true }) { |_| 'value' }
+              subject.expose(:condition_not_met, if: ->(_) { false }) { |_| 'value' }
+            end
+
+            expect(subject.represent({}).value_for(:awesome)).to eq(condition_met: 'value')
+          end
+
+          it 'works with two-argument if condition lambdas' do
+            subject.expose :awesome do
+              subject.expose(:condition_met, if: ->(_, _) { true }) { |_| 'value' }
+              subject.expose(:condition_not_met, if: ->(_, _) { false }) { |_| 'value' }
+            end
+
+            expect(subject.represent({}).value_for(:awesome)).to eq(condition_met: 'value')
+          end
+
+          it 'works with single-argument block exposures' do
+            subject.expose :awesome do
+              subject.expose(:nested) { |obj| obj.class.name }
+            end
+
+            expect(subject.represent({}).value_for(:awesome)).to eq(nested: 'Hash')
+          end
+
           it 'does not represent attributes, declared inside nested exposure, outside of it' do
             subject.expose :awesome do
               subject.expose(:nested) { |_| 'value' }