Fix FmtToIoWriter::write_str to call write_all

Previously, the implementation could silently fail to write some
bytes. Now, the implementation should either write all the bytes or
return an error.

The docs for `std::fmt::Write::write_str` say, "This method can only
succeed if the entire string slice was successfully written, and this
method will not return until all data has been written or an error
occurs." So, `FmtToIoWriter::write_str` must call
`std::io::Write::write_all` instead of `std::io::Write::write` on the
inner writer in order to ensure that all the bytes are written.

src/ser.rs

@@ -867,7 +867,7 @@ where
     W: io::Write,
 {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        if self.writer.write(s.as_bytes()).is_err() {
+        if self.writer.write_all(s.as_bytes()).is_err() {
             return Err(fmt::Error);
         }
         Ok(())