Fix of error when metric lines are loaded as byte array and can't be decoded

When running graphite-beacon, value from graphite metric wasn't loaded properly. Instead of number i got `'Loading error: Type str doesn't support the buffer API'`. 

Value from URL was loaded as byte array, but parsed as string (i.e. `str.split()` was called on it). I've fixed it by decoding value to `UTF-8`. Fix should work in py2 and py3 also if metric value is loaded as string, fix won't break.