URL handling changes for UTF-8

Existing use of `unicode:characters_to_binary was unhelpful`; passing a UTF-8 value resulted in illegal values being passed along to `ibrowse`. Instead, disregard UTF-8 values and simply flatten the iolist, and let `ibrowse` handle them.

This is an intrusive change. Lacking tests, I've run several `riak_test` tests that rely on `rhc`. I experienced difficulties with `mapred_search_switch`, but others that I could easily run worked correctly.