Use WP's array format for get_meta() functions when no $meta_key is provided

Passing only an object_id to a get_meta() function in WP will return an array
of arrays, keyed by meta_keys, each containing a list of meta_values for that
meta_key. BP's get_meta() functions were inconsistent in this regard: some
returned arrays of stdClass, some returned one-d arrays of meta_values. All
returned values that were practically of little use, because they didn't have
enough information about the located data to make it useful.

This changeset aligns BP's get_meta() functions with WP's. We now allow the
return value of get_metadata() to pass through untouched in all cases. Unit
tests have been updated as necessary.

See #5399

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8133 cdf35c40-ae34-48e0-9cc9-0c9da1808c22