Fix incorrect content-header assignment and charset bug

    mime_magic fix for Issue #247:
    Originally mime_magic could assign the charset when it assigned
    content_encoding. The change in 85bb3ed removed the ability of
    mime_magic to set the character set. This reverts that mistake.
    
    Cleanup fix 85bb3ed for bug #153:
    The content-encoding header assignment is removed since the
    intent was a mistake as described in bug #153.