Optionally avoid url encoding params

Hi is it possible to avoid running `URI.encode_query` on the passed in params?

Currently the params are always being run through `URI.encode_query` in https://github.com/edgurgel/httpoison/blob/a4a7877/lib/httpoison/base.ex#L162:

url =
cond do
not Keyword.has_key?(options, :params) -> url
URI.parse(url).query -> url <> "&" <> URI.encode_query(options[:params])
true -> url <> "?" <> URI.encode_query(options[:params])
end

I would like to not url-encode one of the params (because I need to avoid url-encoding the `,` character) for a particular API that I am working with. Do I have any alternatives beside running `URI.encode_query` on my other parameters and joining my non-encoded param manually with a `&`?

Start in lib/httpoison/base.ex at the URL construction around line 162, where options[:params] is passed to URI.encode_query in both query branches. Trace how request options are handled and determine an explicit way to make encoding optional for selected parameters. Done means the requested comma-preserving use case works while the current default parameter encoding remains unchanged.

Written by the indexing model from the issue text.