[BUGFIX beta] Fixes an issue with strict mode

There was previously a bug in Handlebars binding code: it first checked
whether the current context was an object, and if not, it went down a
non-observable path.

However, in sloppy mode, that path was never followed, and as new
features got coalesced into this code path (`#if`, `#with`), it became
increasingly broken. The reason it was never followed in sloppy mode is
that `helper.call(primitive)` was always `typeof this === 'object'`.

When we switched to strict mode, `typeof this` was no longer always
`object`, which meant that the previous, totally broken path was
followed.

This commit maintains the behavior that makes `#if` and `#with` work,
but avoids observing primitives.