avoid FrozenError when params contains frozen value

The error can be fixed by using dupped values.
The original code for testing is implemented by @programmarchy in #1479

Fixes #1478
Closes #1479

lib/sinatra/base.rb

@@ -1089,7 +1089,7 @@ module Sinatra
 
     # Dispatch a request with error handling.
     def dispatch!
-      force_encoding(@params.merge!(@request.params))
+      @params.merge!(@request.params).each { |key, val| @params[key] = force_encoding(val.dup) }
 
       invoke do
         static! if settings.static? && (request.get? || request.head?)

test/middleware_test.rb

@@ -65,4 +65,17 @@ class MiddlewareTest < Minitest::Test
     assert_equal "/FOO", body
     assert_equal "UpcaseMiddleware", response['X-Tests']
   end
+
+  class FreezeMiddleware < MockMiddleware
+    def call(env)
+      req = Rack::Request.new(env)
+      req.update_param('bar', 'baz'.freeze)
+      super
+    end
+  end
+
+  it "works when middleware adds a frozen param" do
+    @app.use FreezeMiddleware
+    get '/Foo'
+  end
 end