Setting meta via PB fails

We can't control whether user meta keys are given to us as lists or binaries. This is a failed test when populating meta via a PB interface.

[error] <0.762.0>@yz_kv:index:161 failed to index object {<<"test">>,<<"tygra">>} with error function_clause because [{string,to_lower,[<<"x-riak-meta-yz-tags">>],[{file,"string.erl"},{line,493}]},{yz_doc,'-get_user_meta/1-lc$^0/1-0-',1,[{file,"src/yz_doc.erl"},{line,139}]},{yz_doc,get_user_meta,1,[{file,"src/yz_doc.erl"},{line,139}]},{yz_doc,extract_tags,1,[{file,"src/yz_doc.erl"},{line,118}]},{yz_doc,make_doc,5,[{file,"src/yz_doc.erl"},{line,64}]},{yz_doc,'-make_docs/4-lc$^0/1-0-',5,[{file,"src/yz_doc.erl"},{line,52}]},{yz_kv,index,7,[{file,"src/yz_kv.erl"},{line,207}]},{yz_kv,index,3,[{file,"src/yz_kv.erl"},{line,148}]}]

I swear I fixed this once before :-/