public void writeWithExistingContent(Scanner existingContent) {
            buffer.setLength(0);
            existingProfileProperties.clear();

            run(existingContent);
        }
Read the existing content of a credentials file, and then make
in-place modification according to the new profiles specified in this
class.
aws-java-sdk-core/src/main/java/com/amazonaws/auth/profile/ProfilesConfigFileWriter.java