Improve specs for Symbol#to_proc

* dry-initializer relies on the arity being -1, see
  #1462
  dry-rb/dry-initializer#52

Author: eregon

spec/ruby/core/symbol/to_proc_spec.rb

@@ -12,19 +12,22 @@
     :to_s.to_proc.call(obj).should == "Received #to_s"
   end
 
+  it "produces a proc with arity -1" do
+    pr = :to_s.to_proc
+    pr.arity.should == -1
+  end
+
   it "raises an ArgumentError when calling #call on the Proc without receiver" do
-    lambda { :object_id.to_proc.call }.should raise_error(ArgumentError)
+    lambda { :object_id.to_proc.call }.should raise_error(ArgumentError, "no receiver given")
   end
 
   it "produces a proc that always returns [[:rest]] for #parameters" do
     pr = :to_s.to_proc
     pr.parameters.should == [[:rest]]
   end
-end
 
-describe "Symbol#to_proc" do
-  before :all do
-    @klass = Class.new do
+  it "passes along the block passed to Proc#call" do
+    klass = Class.new do
       def m
         yield
       end
@@ -33,9 +36,6 @@ def to_proc
         :m.to_proc.call(self) { :value }
       end
     end
-  end
-
-  it "passes along the block passed to Proc#call" do
-    @klass.new.to_proc.should == :value
+    klass.new.to_proc.should == :value
   end
 end