Problem:
Custom UCA collations didn't set the MY_CS_STRNXFRM flag,
which resulted in "prefix_search" method instead of
the required "seq_search".

Problem2: (not metioned in the bug report)
Custom UCA collations didn't also set the MY_CS_UNICODE flag,
so an attempt to compare a column with a custom UCA collation
to another column with a non-Unicode character set led to
the "illegal mix of collation" error.

Fix:
the two missing flags was added into collation initialization.

  Upgrade:

  - All fulltext indexes with custom UCA collations should be rebuilt.

  - Non-fulltext custom UCA indexes should likely be rebuild as well.