Handle jiffy returning an iolist when encoding atts_since query string
If we don't handle it, it throws an error when trying to encode the full URL
string, for example:
```
badarg,[
{mochiweb_util,quote_plus,2,[{file,"src/mochiweb_util.erl"},{line,192}]},
{couch_replicator_httpc,query_args_to_string,2,[{file,"src/couch_replicator_httpc.erl"},{line,421}]},
{couch_replicator_httpc,full_url,2,[{file,"src/couch_replicator_httpc.erl"},{line,413}]},
{couch_replicator_api_wrap,open_doc_revs,6,[{file,"src/couch_replicator_api_wrap.erl"},{line,255}]}
]
```
This is also similar to what we did for open_revs encoding: https://github.com/apache/couchdb/commit/a2d0c4290dde2015e5fb6184696fec3f89c81a4b
1 file changed
