Approx is incorrect for small margins

The following test will incorrectly fail:
REQUIRE(Approx(1.0).margin(0.0000001) != 1.000001);
This is because of the default value for epsilon in the Approx class and the fact that Approx::equalityComparisonImpl will pass if the value is in the range of the margin OR the epsilon values.
Changing the default value of epsilon to zero fixes the bug but doesn't provide the desired default behaviour for Assert.