Replacing string += "." by '.'

[Paolo Carlini]

Benjamin Kosnik wrote:

>>triggered by a recent patch of Paolo, I thought about the implications of 
>>replacing adding single characters like
>> __str += '=';
>>by
>> __str += "=";
>>
>>It seemed to me that the latter is more expensive.
>>
>Absolutely. I was a bit surprised to see this as well.
>
Benjamin, if you agree, I prepare a Changelog entry on behalf of Wolfgang
and commit the patch (after testing, of course).

Ciao,
Paolo.