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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 add_header(self, header):
 """Add a custom HTTP header to the client's request headers"""

        if type(header) is dict:
            self._headers.update(header)
        else:
            raise ValueError(
                "Dictionary expected, got '%s' instead" % type(header)
            )