Fix behavior of Context#resolve when resolving a context function that returns a Chunk

[sethkinast]

There's one edge case that context.resolve() didn't hit.

If you have a function in context (not a generated Dust body function) that returns a chunk, the chunk is not evaluated.

{#foo param="{func_returns_chunk} world"/} {! works because the whole string turns into a generated body function !}
{#foo param=func_that_returns_value /} {! works because the func doesn't return a Chunk !}
{#foo param=func_that_returns_chunk /} {! [object Object] !}

PR fixes this edge case.

[sethkinast]

This must go in some version <= 2.7.0 so dustjs-helpers works.