Header options corresponding to the content type of a HTTP
request body.
Values:
TYPE_UNSPECIFIED (0):
No content type specified.
URL_ENCODED (1):
body is in URL-encoded form. Equivalent to setting the
Content-Type to application/x-www-form-urlencoded in
the HTTP request.
USER_PROVIDED (2):
body is in custom_content_type form. Equivalent to
setting the Content-Type to the contents of
custom_content_type in the HTTP request.
