Use Object instead of String for invalid maxlen/offset values

This way the error message is consistent between different Ruby
platforms.

core/io/pread_spec.rb

@@ -86,11 +86,13 @@
     end
 
     it "raises TypeError if maxlen is not an Integer and cannot be coerced into Integer" do
-      -> { @file.pread("maxlen", 0) }.should raise_error(TypeError, 'no implicit conversion from string')
+      maxlen = Object.new
+      -> { @file.pread(maxlen, 0) }.should raise_error(TypeError, 'no implicit conversion of Object into Integer')
     end
 
     it "raises TypeError if offset is not an Integer and cannot be coerced into Integer" do
-      -> { @file.pread(4, "offset") }.should raise_error(TypeError, 'no implicit conversion of String into Integer')
+      offset = Object.new
+      -> { @file.pread(4, offset) }.should raise_error(TypeError, 'no implicit conversion of Object into Integer')
     end
 
     it "raises ArgumentError for negative values of maxlen" do