handle failure to upcase invalid strings

Some strings cannot be upper cased and raise an ArgumentError instead.
Let's capture this error and log its occurence to the environment via
rack.errors

lib/rack/methodoverride.rb

@@ -26,7 +26,11 @@ def method_override(env)
       req = Request.new(env)
       method = method_override_param(req) ||
         env[HTTP_METHOD_OVERRIDE_HEADER]
-      method.to_s.upcase
+      begin
+        method.to_s.upcase
+      rescue ArgumentError
+        env["rack.errors"].puts "Invalid string for method"
+      end
     end
 
     private

test/spec_methodoverride.rb

@@ -8,7 +8,7 @@ def app
       [200, {"Content-Type" => "text/plain"}, []]
     }))
   end
-  
+
   should "not affect GET requests" do
     env = Rack::MockRequest.env_for("/?_method=delete", :method => "GET")
     app.call env
@@ -23,6 +23,22 @@ def app
     env["REQUEST_METHOD"].should.equal "PUT"
   end
 
+  if RUBY_VERSION >= "1.9"
+    should "set rack.errors for invalid UTF8 _method values" do
+      errors = StringIO.new
+      env = Rack::MockRequest.env_for("/",
+        :method => "POST",
+        :input => "_method=\xBF".force_encoding("ASCII-8BIT"),
+        "rack.errors" => errors)
+
+      app.call env
+
+      errors.rewind
+      errors.read.should.equal "Invalid string for method\n"
+      env["REQUEST_METHOD"].should.equal "POST"
+    end
+  end
+
   should "modify REQUEST_METHOD for POST requests when X-HTTP-Method-Override is set" do
     env = Rack::MockRequest.env_for("/",
             :method => "POST",