public static HttpHeaders parseFromString(String headersString) {
        HttpHeaders headers = new HttpHeaders(headersString);

        if (StringUtils.isNotEmpty(headersString)) {
            try (BufferedReader reader = new BufferedReader(new StringReader(headersString))) {
                String line = reader.readLine();
                while (line != null) {
                    int colonIndex = line.indexOf(':');
                    if (colonIndex > 0) {
                        String headerName = line.substring(0, colonIndex);
                        if (line.length() > colonIndex + 2) {
                            headers.add(headerName, StringUtils.strip(line.substring(colonIndex + 1)));
                        } else {
                            headers.add(headerName, "");
                        }
                        line = reader.readLine();

                    } else {
                        throw new FlowableIllegalArgumentException("Header line '" + line + "' is invalid");
                    }
                }
            } catch (IOException ex) {
                throw new FlowableException("IO exception occurred", ex);
            }
        }

        return headers;
    }
@Test
    void parseFromInvalidStringHeader() {
        assertThatThrownBy(() -> HttpHeaders.parseFromString("Content-Type"))
                .isInstanceOf(FlowableIllegalArgumentException.class)
                .hasMessage("Header line 'Content-Type' is invalid");
    }