py3: convert content-type to bytes
This fixes the exception:
TypeError: %b requires a bytes-like object, or an object that
implements __bytes__, not 'str'
On the line:
ui.debug('Content-Type: %s\n' % content_type)
And it prevents additional exceptions that use the content_type
variable later.
Differential Revision: https://phab.mercurial-scm.org/D2136
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