Change x-gzip to gzip in docs [ci skip]

For more information about GNU zip mime type please check IETF's web site [RFC6713](http://tools.ietf.org/html/rfc6713) or [IANA](http://www.iana.org/assignments/media-types/media-types.xhtml#application)

actionmailer/lib/action_mailer/base.rb

@@ -664,7 +664,7 @@ module ActionMailer
     #
     # You can also specify overrides if you want by passing a hash instead of a string:
     #
-    #  mail.attachments['filename.jpg'] = {mime_type: 'application/x-gzip',
+    #  mail.attachments['filename.jpg'] = {mime_type: 'application/gzip',
     #                                      content: File.read('/path/to/filename.jpg')}
     #
     # If you want to use encoding other than Base64 then you will need to pass encoding
@@ -672,7 +672,7 @@ module ActionMailer
     # data:
     #
     #  file_content = SpecialEncode(File.read('/path/to/filename.jpg'))
-    #  mail.attachments['filename.jpg'] = {mime_type: 'application/x-gzip',
+    #  mail.attachments['filename.jpg'] = {mime_type: 'application/gzip',
     #                                      encoding: 'SpecialEncoding',
     #                                      content: file_content }
     #

guides/source/action_mailer_basics.md

@@ -278,7 +278,7 @@ different, encode your content and pass in the encoded content and encoding in a
     ```ruby
     encoded_content = SpecialEncode(File.read('/path/to/filename.jpg'))
     attachments['filename.jpg'] = {
-      mime_type: 'application/x-gzip',
+      mime_type: 'application/gzip',
       encoding: 'SpecialEncoding',
       content: encoded_content
     }