'Convert a dictionary into one suitable for a HTTP request.
d: a dictionary
Returns: the same dictionary, with x-image-meta added to every key'

@staticmethod
def _dict_to_headers(d):

    h = {}
    for key in d:
        if (key == 'properties'):
            for subkey in d[key]:
                if (d[key][subkey] is None):
                    h[('x-image-meta-property-%s' % subkey)] = ''
                else:
                    h[('x-image-meta-property-%s' % subkey)] = d[key][subkey]
        else:
            h[('x-image-meta-%s' % key)] = d[key]
    return h
