Fix String#lstrip! and String#rstrip! specs to call a valid method.

Author: nirvdrum

spec/ruby/core/string/lstrip_spec.rb

@@ -54,6 +54,6 @@
   it "raises an ArgumentError if the first codepoint is invalid" do
     s = "\xDFabc".force_encoding(Encoding::UTF_8)
     s.valid_encoding?.should be_false
-    -> { s.send(@method) { } }.should raise_error(ArgumentError)
+    -> { s.lstrip! }.should raise_error(ArgumentError)
   end
 end

spec/ruby/core/string/rstrip_spec.rb

@@ -50,6 +50,6 @@
   it "raises an ArgumentError if the last codepoint is invalid" do
     s = "abc\xDF".force_encoding(Encoding::UTF_8)
     s.valid_encoding?.should be_false
-    -> { s.send(@method) { } }.should raise_error(ArgumentError)
+    -> { s.rstrip! }.should raise_error(ArgumentError)
   end
 end