The send_error method always sends a Content-Type of 'text/html'.  Other
content types are both possible and desirable.  For example, someone
writing a REST server might wish to send XML error messages.  Following
the example of DEFAULT_ERROR_MESSAGE, I propose adding the following in
the appropriate places:

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> # Default error content-type
> DEFAULT_ERROR_TYPE = 'text/html'
>
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<         self.send_header("Content-Type", "text/html")
---
>         self.send_header("Content-Type", error_message_type)
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>     error_message_type = DEFAULT_ERROR_TYPE