[lmi] Reading an empty file into a string

[Greg Chicares]

We've encountered an 'istream_to_string.hpp' problem with msvc,
in this code:

    if(is.rdbuf()->in_avail())
        {
        oss << is.rdbuf();
        }

because msvc's in_avail() always returns zero, as is apparently
permitted:

http://groups.google.com/group/comp.lang.c++.moderated/msg/f3788f581b712d1e

It still seems desirable for this function template to produce an
empty string from an empty file (more precisely, from an fstream
corresponding to an empty file), without throwing an exception.
Is there a tidy and reliable way to do that?

I added a unit test on 20080921T1309Z to make this easier to
investigate, and also to compare performance of two methods:
  Method 0: construct string from istreambuf_iterator pair
  Method 1: extract from rdbuf into stringstream; use its str()
of which the latter is used in production. Sample timings for a
30627-byte file:

MinGW gcc-3.4.4:
  Method 0: 5.776e-004 s =     577611 ns, mean of 100 iterations
  Method 1: 3.307e-004 s =     330730 ns, mean of 100 iterations

como-4.3.3:
  Method 0: 2.030e-03  s =    2029539 ns, mean of 100 iterations
  Method 1: 3.672e-03  s =    3671666 ns, mean of 100 iterations