Partially revert "All data passed around in Horde internally should be UTF-8."

Partially revert "All data passed around in Horde internally should be UTF-8."

This partially reverts commit e60da622060274282dd92c2d41c4c0dce4724dd6.

Horde_Text_Filter is not part of Horde the application - so there is no
guarantee that text is in UTF-8.  In fact, much of the usefulness of
this package is it does the necessary charset conversion as needed.

Example: for Horde_Mime_Part objects created from message data, the text
may be in a different charset than UTF-8.  This is perfectly fine -
unnecessary charset conversion may just be a waste of time.

This fixes incorrect character display in IMP (at least for the few
messages I have tested so far).