Fix few issues reported by the newly added rule

spec/ameba/ast/scope_spec.cr

@@ -135,14 +135,16 @@ module Ameba::AST
       scope = Scope.new as_node("foo = 1")
       scope.add_variable Crystal::Var.new "foo"
       scope.assign_variable("foo", Crystal::Var.new "foo")
-      scope.find_variable("foo").not_nil!.assignments.size.should eq 1
+      var = scope.find_variable("foo").should_not be_nil
+      var.assignments.size.should eq 1
     end
 
     it "does not create the assignment if variable is wrong" do
       scope = Scope.new as_node("foo = 1")
       scope.add_variable Crystal::Var.new "foo"
       scope.assign_variable("bar", Crystal::Var.new "bar")
-      scope.find_variable("foo").not_nil!.assignments.size.should eq 0
+      var = scope.find_variable("foo").should_not be_nil
+      var.assignments.size.should eq 0
     end
   end
 

spec/ameba/formatter/todo_formatter_spec.cr

@@ -86,8 +86,8 @@ module Ameba
         s1.add_issue DummyRule.new, {2, 2}, "message1"
         s2.add_issue DummyRule.new, {2, 2}, "message2"
 
-        file = formatter.finished([s1, s2])
-        content = File.read(file.not_nil!.path)
+        file = formatter.finished([s1, s2]).should_not be_nil
+        content = File.read(file.path)
         content.should contain <<-CONTENT
         # Problems found: 3
         # Run `ameba --only Ameba/DummyRule` for details

src/ameba/rule/lint/percent_array.cr

@@ -41,12 +41,12 @@ module Ameba::Rule::Lint
         when .string_array_start?, .symbol_array_start?
           start_token = token.dup
         when .string?
-          if start_token && issue.nil?
-            issue = array_entry_invalid?(token.value, start_token.not_nil!.raw)
+          if (_start = start_token) && !issue
+            issue = array_entry_invalid?(token.value, _start.raw)
           end
         when .string_array_end?
-          if issue
-            issue_for start_token.not_nil!, issue.not_nil!
+          if (_start = start_token) && (_issue = issue)
+            issue_for _start, _issue
           end
           issue = start_token = nil
         end

src/ameba/rule/lint/redundant_with_object.cr

@@ -37,8 +37,8 @@ module Ameba::Rule::Lint
     def test(source, node : Crystal::Call)
       return if node.name != "each_with_object" ||
                 node.args.size != 1 ||
-                node.block.nil? ||
-                with_index_arg?(node.block.not_nil!)
+                !(block = node.block) ||
+                with_index_arg?(block)
 
       issue_for node.name_location, node.name_end_location, MSG
     end