sql: fix CAST() from STRING to INTEGER

Prior to this patch, STRING, which contains the DOUBLE value,
could be cast to INTEGER. This was done by converting STRING to
DOUBLE and then converting this DOUBLE value to INTEGER. This may
affect the accuracy of CAST(), so it was forbidden.

Before patch:
box.execute("SELECT CAST('111.1' as INTEGER);")
Result: 111

After patch:
box.execute("SELECT CAST('1.1' as INTEGER);")
Result: 'Type mismatch: can not convert 1.1 to integer'

box.execute("SELECT CAST('1.0' as INTEGER);")
Result: 'Type mismatch: can not convert 1.0 to integer'

box.execute("SELECT CAST('1.' as INTEGER);")
Result: 'Type mismatch: can not convert 1. to integer'