encode string request body with utf-8 not the platform default

[alhudz]

Repro: build a body with Request.Body.create(String) on a JVM whose default charset is not UTF-8 (-Dfile.encoding=ISO-8859-1) and send a non-ASCII payload such as "é"; the bytes on the wire are the ISO-8859-1 encoding, not the UTF-8 the rest of feign and the declared Content-Type assume.
Cause: create(String) encodes with data.getBytes(), the platform-default charset, and leaves the body encoding null so it is flagged binary. Every other body path defaults to UTF-8 (RequestTemplate.body(String) and the create(String, Charset) overload), so the result is deployment-dependent and the dropped charset means asString()/charset() cannot recover the text.
Fix: delegate to create(data, UTF_8) so the bytes are deterministic UTF-8 and the charset is recorded, matching the sibling overload.