Avoid explicitly resetting a std::[experimental::]optional

[bmerry]

Now that #851 has removed all multiple uses of a caster, it can just use
the default-constructed value with needing a reset. This fixes two
issues:

1. With std::experimental::optional (at least under GCC 5.4), the = {}
   would construct an instance of the optional type and then move-assign
   it, which fails if the value type isn't move-assignable.

2. With older versions of Boost, the = {} could fail because it is
   ambiguous, allowing construction of either boost::none or the value
   type.

[bmerry]

This is intended as an alternative to #850.

[dean0x7d]

Thanks for simplifying this! Looks good together with the updated casters. The single-use rule is manually enforced right now, but it should become a compiler-enforced feature with #864.