[bugfix] Use const references in Hybrid::Imp::equals()

If T2/T2 are not defined by 'const T1&', they will themselfes
be of reference type. Hence T2::value does not exist and the
equals() overload for the static evaluation is not selected.
Then the dynamic equals() overload is used. This will give the
same result, but the compiler will instantiate the if branch
with the identity if if the result is 'false'.

This is what lead to a failing test in dune-tnnmg when looping
over a MultitypeMatrix because the if-branch is used with the
wrong type.