Comparisons of basic_string_views with custom char_traits do not print out the string values

When I run the following tests
```c++
struct custom_char_traits : std::char_traits {};

BOOST_AUTO_TEST_CASE(custom_char_traits_compare)
{
using sv = std::basic_string_view;
sv sv1 = "sv1";
sv sv2 = "sv2";
BOOST_TEST(sv1 == sv2);
}

BOOST_AUTO_TEST_CASE(std_char_traits_compare)
{
using sv = std::basic_string_view>;
sv sv1 = "sv1";
sv sv2 = "sv2";
BOOST_TEST(sv1 == sv2);
}
```
I get the following output
```
*** 2 failures are detected in the test module "string view comparison"
Running 2 test cases...
example.cpp(14): error: in "custom_char_traits_compare": check sv1 == sv2 has failed
example.cpp(22): error: in "std_char_traits_compare": check sv1 == sv2 has failed [sv1 != sv2]
```

The error message for `std_char_traits_compare` contains the string values, but the error for `custom_char_traits_compare` does not.
Boost Test should display the compared strings regardless of what traits they have.