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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_header(self, key):
 """Returns the requested header, or an empty string if the header is not set. Keyword arguments: key -- The header name. It will be canonicalized before use. """

        key = canonicalize_header(key)
        if key in self.header:
            return self.header[key]
        return ''