set default content type (#92)

Author: KazuCocoa

lib/appium_lib_core/common/base/http_default.rb

@@ -5,7 +5,11 @@ module Core
     class Base
       module Http
         class Default < Selenium::WebDriver::Remote::Http::Default
-          DEFAULT_HEADERS = { 'Accept' => CONTENT_TYPE, 'User-Agent' => "appium/ruby_lib_core/#{VERSION}" }.freeze
+          DEFAULT_HEADERS = {
+            'Accept' => CONTENT_TYPE,
+            'Content-Type' => "#{CONTENT_TYPE}; charset=UTF-8",
+            'User-Agent' => "appium/ruby_lib_core/#{VERSION}"
+          }.freeze
         end
       end
     end