test: remove rubocop security warnings from TestCase

- change implicit Kernel.open to ::File.open
- replace `eval` with `define_method`

lib/mechanize/test_case.rb

@@ -230,9 +230,9 @@ def request(req, *data, &block)
     else
       filename = "htdocs#{path.gsub(/[^\/\\.\w\s]/, '_')}"
       unless PAGE_CACHE[filename]
-        open("#{Mechanize::TestCase::TEST_DIR}/#{filename}", 'rb') { |io|
+        ::File.open("#{Mechanize::TestCase::TEST_DIR}/#{filename}", 'rb') do |io|
           PAGE_CACHE[filename] = io.read
-        }
+        end
       end
 
       res.body = PAGE_CACHE[filename]

lib/mechanize/test_case/gzip_servlet.rb

@@ -13,7 +13,7 @@ def do_GET(req, res)
     end
 
     if name = req.query['file'] then
-      open "#{TEST_DIR}/htdocs/#{name}" do |io|
+      ::File.open("#{TEST_DIR}/htdocs/#{name}") do |io|
         string = String.new
         zipped = StringIO.new string, 'w'
         Zlib::GzipWriter.wrap zipped do |gz|

lib/mechanize/test_case/verb_servlet.rb

@@ -1,11 +1,9 @@
 class VerbServlet < WEBrick::HTTPServlet::AbstractServlet
   %w[HEAD GET POST PUT DELETE].each do |verb|
-    eval <<-METHOD
-      def do_#{verb}(req, res)
-        res.header['X-Request-Method'] = #{verb.dump}
-        res.body = #{verb.dump}
-      end
-    METHOD
+    define_method "do_#{verb}" do |req, res|
+      res.header['X-Request-Method'] = verb
+      res.body = verb
+    end
   end
 end