rblib: make case-checking Unicode-aware

There is a function in rblib for checking that a given string is not (almost) entirely
upper-case or lower-case.  (This is used by Alaveteli, for example, as an indication
that the user is trying to submit a poorly written request.)  Unfortunately, this only
counted A-Z as capital letters and a-z as lower case letters.  This pull 
changes this behaviour to count non-ASCII upper-case and lower-case letters as
well - there are thousands of these in Unicode - and improves the tests for these
functions.