Remove unnecessary string allocation
Remove unnecessary string allocation

When streaming the chunked GridFS file, instead of assembling the
contents into a new string, we can just return the already allocated
string. Normally this wouldn't matter, but since these strings are parts
of a file they're relatively large (default is 255 KB, but people can
set it to more).

mongodb/mongo-ruby-driver,brandonblack/mongo-ruby-driver,wandenberg/mongo-ruby-driver,kay-kim/mongo-ruby-driver,estolfo/mongo-ruby-driver,mongodb/mongo-ruby-driver,jonhyman/mongo-ruby-driver,jonhyman/mongo-ruby-driver,estolfo/mongo-ruby-driver,saghm/mongo-ruby-driver,mongodb/mongo-ruby-driver,kay-kim/mongo-ruby-driver,saghm/mongo-ruby-driver