bug in export bibtex

[humufr at yahoo.fr]

	Hello,

I found a bug in the export in bibtex

I have a field like 

number = 12-13,

or:

volume = 12 (Pt 10),

and this is not correct, Bibtex can't compile it. The field must be 

number={12-13},

or:

volume = {12 (Pt 10)},

so perhaps that will be better to put 
{ and } at the begin and end of all the fields.  

That will avoid this problems (I think) with bibtex.

Thanks,

N.