code: public static InputStream toInputStream(Publisher<String> publisher, Charset charset) {
          return FlowableStringInputStream.createInputStream(publisher, charset);
      }
docs: Returns an {@link InputStream} that offers the concatenated String data
  emitted by a subscription to the given publisher using the given character set.
  
  @param publisher the source of the String data
  @param charset the character set of the bytes to be read in the InputStream
  @return offers the concatenated String data emitted by a subscription to
  the given publisher using the given character set