remove unicode:characters_to_list/1 conversions

Calls to unicode:characters_to_list/1 in functions such as rhc:make_url/4
prevent callers from passing in any unicode binaries already converted to
UTF-8, since unicode:characters_to_list/1 converts the data back to full
unicode values. Since these values can exceed the range of a byte, trying
to send them over a socket as part of an HTTP request fails with einval.

These changes allow UTF-8 binaries passed from the caller to work
correctly.