dateFromHttpDateString: fixes

[bazineta]

In datefromHttpDateString, the following two lines:

    if(((size_t)convertedCount != stringLength) || (usedBytes < 0L)) { return(NULL); }
    stringBuffer[convertedCount] = 0;

should instead be:

    if(((size_t)convertedCount != stringLength) || (usedBytes < 0L)) { return(nil); }
    stringBuffer[usedBytes] = '\0';

[steipete]

Ah, you're right. John must have gotten that wrong. characters can be less than bytes used.