BUGFIX to Bio::DB::GenericWebAgent that allows -file to accept more perl-like write-append type data

I was following examples in the EUtilities Cookbook (http://www.bioperl.org/wiki/HOWTO:EUtilities_Cookbook) to try and fetch genbank sequences.  I wanted to append to an output file, but this did not work so I made the following fix.

Bio::DB::EUtilities uses as its base class Bio::DB::GenericWebAgent, so this is a minor bugfix made to Bio::DB::GenericWebAgent that allows "-file to accept more perl-like write-append type data."

The following will write to a file:
$factory->get_Response(-file => 'file.gb');
$factory->get_Response(-file => '>newFile.gb');

The following will append to a file:
$factory->get_Response(-file => '>>appendedFile.gb');