sal: do not ignore conversion flags for RTL_TEXTENCODING_ASCII_US

Keep the fast path fast, fall back to the text encoder in case there's a
fly in the ointment.

Change-Id: I94507856a7f3170f770adb741aa1e282d0d2400c
sal: it is impossible to have non-ASCII char here

...because rtl_ImplGetFastUTF8UnicodeLen has already checked that.

Change-Id: I17f2b80f374073934a8f0b1a97099d4dec89ce4e
filter: use more appropriate conversion from ASCII for arbitrary input

... with unknown encoding, mapping non-ASCII chars to something that
doesn't cause trouble (in FilterDetect::detect()).

Change-Id: Ibf2a2e2fd7c0c00e55042d2ccad173fab7a1b0bd