Root cause for this bug is that the optimizer try to detect&
optimize the special case:
      
'<field>  BETWEEN c1 AND c1' and handle this as the condition '<field>  = c1'
            
This was implemented inside add_key_field(.. *field, *value[]...)
which assumed field to refer key Field, and value[] to refer a [low...high]
constant pair. value[0] and value[1] was then compared for equality.
            
In a 'normal' BETWEEN condition of the form '<field>  BETWEEN val1 and val2' the
BETWEEN operation is represented with an argementlist containing the
values [<field>, val1, val2] - add_key_field() is then called with
parameters field=<field>, *value=val1.
            
However, if the BETWEEN predicate specified:
            
 1)  '<const1>  BETWEEN<const2>  AND<field>
            
the 'field' and 'value' arguments to add_key_field() had to be swapped.
This was implemented by trying to cheat add_key_field() to handle it like:
            
 2) '<const1>  GE<const2>  AND<const1>  LE<field>'
            
As we didn't really replace the BETWEEN operation with 'ge' and 'le',
add_key_field() still handled it as a 'BETWEEN' and compared the (swapped)
arguments<const1>  and<const2>  for equality. If they was equal, the
condition 1) was incorrectly 'optimized' to:
            
 3) '<field>  EQ <const1>'
            
This fix moves this optimization of '<field>  BETWEEN c1 AND c1' into
add_key_fields() which then calls add_key_equal_fields() to collect 
key equality / comparison for the key fields in the BETWEEN condition.