fix: use utf8 encoding for text response type

[Vinzent03]

close supabase/supabase-flutter#367

From Response.body:

/// The body of the response as a string.
///
/// This is converted from [bodyBytes] using the charset parameter of the
/// Content-Type header field, if available. If it's unavailable or if the
/// encoding name is unknown, [latin1] is used by default, as per
/// [RFC 2616][].
///
/// [RFC 2616]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html

So we can either always use utf8(as changed in this pr) or the user has to specify their headers correctly.

[dshukertjr]

Thanks for the fix and the cleanup! Could we bump the version in this PR as well?

[Vinzent03]

@dshukertjr Sure, done!

[dshukertjr]

Thanks for the amazing work!