Fix all inputs containing non-ASCII characters were garbled

As of network-2.4.1.0, Network.URI.unEscapeString converts input bytes to a UTF-8 string, although this library uses it to just unescape a percent-encoded string. As a result, it converts an input to a Unicode string then truncates all characters in the string to 8 bits by using Data.ByteString.Lazy.Char8. I copied unEscapeString from the old network library that just unescape a percent-encoded string.