Fix Padrino::Mailer::Mime.mime_type

If we called #encoded or #to_s of mail message object,
charset will be append to the 'content_type' of the mail-header.
In this state, Messge#content_type return incorrect value.

```ruby
message = Sample::App._padrino_mailer::Message.new { content_type 'text/html'; body 'I wanna eat sushi.' }
message.header['content_type'].to_s # => "text/html"
message.content_type #=> :html

message.encoded # or #to_s

message.header['content_type'].to_s #=> "text/html; charset=UTF-8"
message.content_type #=> :plain
```

This PR will fix it.

```ruby
message = Sample::App._padrino_mailer::Message.new { content_type 'text/html'; body 'I wanna eat sushi.' }
message.header['content_type'].to_s # => "text/html"
message.content_type #=> :html

message.encoded # or #to_s

message.header['content_type'].to_s #=> "text/html; charset=UTF-8"
message.content_type #=> :html
```